[RFC,v2,0/2] readmirror feature
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Message ID 20190826090438.7044-1-anand.jain@oracle.com
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Anand Jain Aug. 26, 2019, 9:04 a.m. UTC
Function call chain  __btrfs_map_block()->find_live_mirror() uses
thread pid to determine the %mirror_num when the mirror_num=0.

This patch introduces a framework so that we can add policies to determine
the %mirror_num. And also adds the devid as the readmirror policy.

The new property is stored as an item in the device tree as show below.
    (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)

To be able to set and get this new property also introduces new ioctls
BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
is defined as
        struct btrfs_ioctl_readmirror_args {
                __u64 type; /* RW */
                __u64 device_bitmap; /* RW */
        }

An usage example as follows:
        btrfs property set /btrfs readmirror devid:1,3
        btrfs property get /btrfs readmirror
          readmirror devid:1 3
        btrfs property set /btrfs readmirror ""
        btrfs property get /btrfs readmirror
          readmirror default

This patchset has been tested completely, however marked as RFC for the 
following reasons and comments on them (or any other) are appreciated as
usual.
 . The new objectid is defined as
      #define BTRFS_READMIRROR_OBJECTID -1ULL
   Need consent we are fine to use this value, and with this value it
   shall be placed just before the DEV_STATS_OBJECTID item which is more
   frequently used only during the device errors.

.  I am using a u64 bitmap to represent the devices id, so the max device
   id that we could represent is 63, its a kind of limitation which should
   be addressed before integration, I wonder if there is any suggestion?
   Kindly note that, multiple ioctls with each time representing a set of
   device(s) is not a choice because we need to make sure the readmirror
   changes happens in a commit transaction.

v1->RFC v2:
  . Property is stored as a dev-tree item instead of root inode extended
    attribute.
  . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
  . Changed format specifier from devid1,2,3.. to devid:1,2,3..

RFC->v1:
  Drops pid as one of the readmirror policy choices and as usual remains
  as default. And when the devid is reset the readmirror policy falls back
  to pid.
  Drops the mount -o readmirror idea, it can be added at a later point of
  time.
  Property now accepts more than 1 devid as readmirror device. As shown
  in the example above.

Anand Jain (1):
  btrfs: add readmirror framework and policy devid

 fs/btrfs/ctree.h                |   3 +
 fs/btrfs/disk-io.c              |   9 ++
 fs/btrfs/ioctl.c                | 108 ++++++++++++++++++++++++
 fs/btrfs/transaction.c          |   3 +
 fs/btrfs/volumes.c              | 145 +++++++++++++++++++++++++++++++-
 fs/btrfs/volumes.h              |   9 +-
 include/uapi/linux/btrfs.h      |  15 +++-
 include/uapi/linux/btrfs_tree.h |  11 +++
 8 files changed, 300 insertions(+), 3 deletions(-)

Anand Jain (1):
  btrfs_progs: add readmirror property and ioctl to set get

 ioctl.h | 14 +++++++++
 props.c | 98 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)

Comments

David Sterba Sept. 11, 2019, 4:37 p.m. UTC | #1
On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> Function call chain  __btrfs_map_block()->find_live_mirror() uses
> thread pid to determine the %mirror_num when the mirror_num=0.
> 
> This patch introduces a framework so that we can add policies to determine
> the %mirror_num. And also adds the devid as the readmirror policy.
> 
> The new property is stored as an item in the device tree as show below.
>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> 
> To be able to set and get this new property also introduces new ioctls
> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> is defined as
>         struct btrfs_ioctl_readmirror_args {
>                 __u64 type; /* RW */
>                 __u64 device_bitmap; /* RW */
>         }

I don't remember if there was a suggestion to use ioctls for read
mirror, but the property interface should be sufficient. Besides this
ioctl interafce is quite an anti-pattern: narrow use, non-extensible
structure, alternative and more convenient interface already available.

> An usage example as follows:
>         btrfs property set /btrfs readmirror devid:1,3
>         btrfs property get /btrfs readmirror
>           readmirror devid:1 3
>         btrfs property set /btrfs readmirror ""
>         btrfs property get /btrfs readmirror
>           readmirror default
> 
> This patchset has been tested completely, however marked as RFC for the 
> following reasons and comments on them (or any other) are appreciated as
> usual.
>  . The new objectid is defined as
>       #define BTRFS_READMIRROR_OBJECTID -1ULL
>    Need consent we are fine to use this value, and with this value it
>    shall be placed just before the DEV_STATS_OBJECTID item which is more
>    frequently used only during the device errors.

-1 can be considered a special value in other cases, not necessarily
here but if the ordering of items is the only reason I'd say no. The
keys/items will most likely land in the same node so there's no point
forcing the order.

> .  I am using a u64 bitmap to represent the devices id, so the max device
>    id that we could represent is 63, its a kind of limitation which should
>    be addressed before integration, I wonder if there is any suggestion?

Uh 63, so now an implementation detail is posing a global limit? That
sounds backwards.

>    Kindly note that, multiple ioctls with each time representing a set of
>    device(s) is not a choice because we need to make sure the readmirror
>    changes happens in a commit transaction.

I believe this can be guaranteed by the properties interface, ie. single
value gets processed at once and with some locking around the state of
devices can be updated atomically.

The open question is still how to store the readmirror property
per-device, it could be either an item or bit inside the device
structure.

Besides the interface, I'm kind of missing the usecase description what
is expected from the read mirror policies and how they could affect
various scenarios. Maybe it was in some of the previous iterations, it's
hard too track everything so this should be part of the cover letter (or
at leat linked if it's too much text).
Josef Bacik Sept. 11, 2019, 6:42 p.m. UTC | #2
On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> Function call chain  __btrfs_map_block()->find_live_mirror() uses
> thread pid to determine the %mirror_num when the mirror_num=0.
> 
> This patch introduces a framework so that we can add policies to determine
> the %mirror_num. And also adds the devid as the readmirror policy.
> 
> The new property is stored as an item in the device tree as show below.
>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> 
> To be able to set and get this new property also introduces new ioctls
> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> is defined as
>         struct btrfs_ioctl_readmirror_args {
>                 __u64 type; /* RW */
>                 __u64 device_bitmap; /* RW */
>         }
> 
> An usage example as follows:
>         btrfs property set /btrfs readmirror devid:1,3
>         btrfs property get /btrfs readmirror
>           readmirror devid:1 3
>         btrfs property set /btrfs readmirror ""
>         btrfs property get /btrfs readmirror
>           readmirror default
> 
> This patchset has been tested completely, however marked as RFC for the 
> following reasons and comments on them (or any other) are appreciated as
> usual.
>  . The new objectid is defined as
>       #define BTRFS_READMIRROR_OBJECTID -1ULL
>    Need consent we are fine to use this value, and with this value it
>    shall be placed just before the DEV_STATS_OBJECTID item which is more
>    frequently used only during the device errors.
> 
> .  I am using a u64 bitmap to represent the devices id, so the max device
>    id that we could represent is 63, its a kind of limitation which should
>    be addressed before integration, I wonder if there is any suggestion?
>    Kindly note that, multiple ioctls with each time representing a set of
>    device(s) is not a choice because we need to make sure the readmirror
>    changes happens in a commit transaction.
> 
> v1->RFC v2:
>   . Property is stored as a dev-tree item instead of root inode extended
>     attribute.
>   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
>   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> 
> RFC->v1:
>   Drops pid as one of the readmirror policy choices and as usual remains
>   as default. And when the devid is reset the readmirror policy falls back
>   to pid.
>   Drops the mount -o readmirror idea, it can be added at a later point of
>   time.
>   Property now accepts more than 1 devid as readmirror device. As shown
>   in the example above.
> 

This is a lot of infrastructure to just change which mirror we read to based on
some arbitrary user policy.  I assume this is to solve the case where you have
slow and fast disks, so you can always read from the fast disk?  And then it's
only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
SSD and a normal disk?  I'm not seeing a point to this much code for one
particular obscure setup.  Thanks,

Josef
Eli V Sept. 11, 2019, 7:13 p.m. UTC | #3
On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> > Function call chain  __btrfs_map_block()->find_live_mirror() uses
> > thread pid to determine the %mirror_num when the mirror_num=0.
> >
> > This patch introduces a framework so that we can add policies to determine
> > the %mirror_num. And also adds the devid as the readmirror policy.
> >
> > The new property is stored as an item in the device tree as show below.
> >     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> >
> > To be able to set and get this new property also introduces new ioctls
> > BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> > is defined as
> >         struct btrfs_ioctl_readmirror_args {
> >                 __u64 type; /* RW */
> >                 __u64 device_bitmap; /* RW */
> >         }
> >
> > An usage example as follows:
> >         btrfs property set /btrfs readmirror devid:1,3
> >         btrfs property get /btrfs readmirror
> >           readmirror devid:1 3
> >         btrfs property set /btrfs readmirror ""
> >         btrfs property get /btrfs readmirror
> >           readmirror default
> >
> > This patchset has been tested completely, however marked as RFC for the
> > following reasons and comments on them (or any other) are appreciated as
> > usual.
> >  . The new objectid is defined as
> >       #define BTRFS_READMIRROR_OBJECTID -1ULL
> >    Need consent we are fine to use this value, and with this value it
> >    shall be placed just before the DEV_STATS_OBJECTID item which is more
> >    frequently used only during the device errors.
> >
> > .  I am using a u64 bitmap to represent the devices id, so the max device
> >    id that we could represent is 63, its a kind of limitation which should
> >    be addressed before integration, I wonder if there is any suggestion?
> >    Kindly note that, multiple ioctls with each time representing a set of
> >    device(s) is not a choice because we need to make sure the readmirror
> >    changes happens in a commit transaction.
> >
> > v1->RFC v2:
> >   . Property is stored as a dev-tree item instead of root inode extended
> >     attribute.
> >   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> >   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> >
> > RFC->v1:
> >   Drops pid as one of the readmirror policy choices and as usual remains
> >   as default. And when the devid is reset the readmirror policy falls back
> >   to pid.
> >   Drops the mount -o readmirror idea, it can be added at a later point of
> >   time.
> >   Property now accepts more than 1 devid as readmirror device. As shown
> >   in the example above.
> >
>
> This is a lot of infrastructure to just change which mirror we read to based on
> some arbitrary user policy.  I assume this is to solve the case where you have
> slow and fast disks, so you can always read from the fast disk?  And then it's
> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> SSD and a normal disk?  I'm not seeing a point to this much code for one
> particular obscure setup.  Thanks,
>
> Josef

Not commenting on the code itself, but as a user I see this SSD RAID1
acceleration as a future much have feature. It's only obscure at the
moment because we don't have code to take advantage of it. But on
large btrfs filesystems with hundreds of GB of metadata, like I have
for backups, the usability of the filesystem is dramatically improved
having the metadata on an SSD( though currently only half of the time
due to the even/odd pid distribution.)
Josef Bacik Sept. 11, 2019, 7:16 p.m. UTC | #4
On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
> >
> > On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> > > Function call chain  __btrfs_map_block()->find_live_mirror() uses
> > > thread pid to determine the %mirror_num when the mirror_num=0.
> > >
> > > This patch introduces a framework so that we can add policies to determine
> > > the %mirror_num. And also adds the devid as the readmirror policy.
> > >
> > > The new property is stored as an item in the device tree as show below.
> > >     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> > >
> > > To be able to set and get this new property also introduces new ioctls
> > > BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> > > is defined as
> > >         struct btrfs_ioctl_readmirror_args {
> > >                 __u64 type; /* RW */
> > >                 __u64 device_bitmap; /* RW */
> > >         }
> > >
> > > An usage example as follows:
> > >         btrfs property set /btrfs readmirror devid:1,3
> > >         btrfs property get /btrfs readmirror
> > >           readmirror devid:1 3
> > >         btrfs property set /btrfs readmirror ""
> > >         btrfs property get /btrfs readmirror
> > >           readmirror default
> > >
> > > This patchset has been tested completely, however marked as RFC for the
> > > following reasons and comments on them (or any other) are appreciated as
> > > usual.
> > >  . The new objectid is defined as
> > >       #define BTRFS_READMIRROR_OBJECTID -1ULL
> > >    Need consent we are fine to use this value, and with this value it
> > >    shall be placed just before the DEV_STATS_OBJECTID item which is more
> > >    frequently used only during the device errors.
> > >
> > > .  I am using a u64 bitmap to represent the devices id, so the max device
> > >    id that we could represent is 63, its a kind of limitation which should
> > >    be addressed before integration, I wonder if there is any suggestion?
> > >    Kindly note that, multiple ioctls with each time representing a set of
> > >    device(s) is not a choice because we need to make sure the readmirror
> > >    changes happens in a commit transaction.
> > >
> > > v1->RFC v2:
> > >   . Property is stored as a dev-tree item instead of root inode extended
> > >     attribute.
> > >   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> > >   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> > >
> > > RFC->v1:
> > >   Drops pid as one of the readmirror policy choices and as usual remains
> > >   as default. And when the devid is reset the readmirror policy falls back
> > >   to pid.
> > >   Drops the mount -o readmirror idea, it can be added at a later point of
> > >   time.
> > >   Property now accepts more than 1 devid as readmirror device. As shown
> > >   in the example above.
> > >
> >
> > This is a lot of infrastructure to just change which mirror we read to based on
> > some arbitrary user policy.  I assume this is to solve the case where you have
> > slow and fast disks, so you can always read from the fast disk?  And then it's
> > only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> > SSD and a normal disk?  I'm not seeing a point to this much code for one
> > particular obscure setup.  Thanks,
> >
> > Josef
> 
> Not commenting on the code itself, but as a user I see this SSD RAID1
> acceleration as a future much have feature. It's only obscure at the
> moment because we don't have code to take advantage of it. But on
> large btrfs filesystems with hundreds of GB of metadata, like I have
> for backups, the usability of the filesystem is dramatically improved
> having the metadata on an SSD( though currently only half of the time
> due to the even/odd pid distribution.)

But that's different from a mirror.  100% it would be nice to say "put my
metadata on the ssd, data elsewhere".  That's not what this patch is about, this
patch is specifically about changing which drive we choose in a mirrored setup,
which is super unlikely to mirror a SSD with a slow drive, cause it's just going
to be slow no matter what.  Sure we could make it so reads always go to the SSD,
but we can accomplish that by just adding a check for nonrotational in the code,
and then we don't have to encode all this nonsense in the file system.  Thanks,

Josef
Anand Jain Sept. 12, 2019, 7:41 a.m. UTC | #5
Thanks for the comments. More below.

On 12/9/19 3:16 AM, Josef Bacik wrote:
> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>>
>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
>>>> thread pid to determine the %mirror_num when the mirror_num=0.
>>>>
>>>> This patch introduces a framework so that we can add policies to determine
>>>> the %mirror_num. And also adds the devid as the readmirror policy.
>>>>
>>>> The new property is stored as an item in the device tree as show below.
>>>>      (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
>>>>
>>>> To be able to set and get this new property also introduces new ioctls
>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
>>>> is defined as
>>>>          struct btrfs_ioctl_readmirror_args {
>>>>                  __u64 type; /* RW */
>>>>                  __u64 device_bitmap; /* RW */
>>>>          }
>>>>
>>>> An usage example as follows:
>>>>          btrfs property set /btrfs readmirror devid:1,3
>>>>          btrfs property get /btrfs readmirror
>>>>            readmirror devid:1 3
>>>>          btrfs property set /btrfs readmirror ""
>>>>          btrfs property get /btrfs readmirror
>>>>            readmirror default
>>>>
>>>> This patchset has been tested completely, however marked as RFC for the
>>>> following reasons and comments on them (or any other) are appreciated as
>>>> usual.
>>>>   . The new objectid is defined as
>>>>        #define BTRFS_READMIRROR_OBJECTID -1ULL
>>>>     Need consent we are fine to use this value, and with this value it
>>>>     shall be placed just before the DEV_STATS_OBJECTID item which is more
>>>>     frequently used only during the device errors.
>>>>
>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
>>>>     id that we could represent is 63, its a kind of limitation which should
>>>>     be addressed before integration, I wonder if there is any suggestion?
>>>>     Kindly note that, multiple ioctls with each time representing a set of
>>>>     device(s) is not a choice because we need to make sure the readmirror
>>>>     changes happens in a commit transaction.
>>>>
>>>> v1->RFC v2:
>>>>    . Property is stored as a dev-tree item instead of root inode extended
>>>>      attribute.
>>>>    . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
>>>>    . Changed format specifier from devid1,2,3.. to devid:1,2,3..
>>>>
>>>> RFC->v1:
>>>>    Drops pid as one of the readmirror policy choices and as usual remains
>>>>    as default. And when the devid is reset the readmirror policy falls back
>>>>    to pid.
>>>>    Drops the mount -o readmirror idea, it can be added at a later point of
>>>>    time.
>>>>    Property now accepts more than 1 devid as readmirror device. As shown
>>>>    in the example above.
>>>>
>>>
>>> This is a lot of infrastructure

   Ok. Any idea on a better implementation?
   How about extended attribute approach? v1 patches proposed
   it, but it abused the extended attribute as commented here [1]
   and v2 got changed to an item-key.

[1]
https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/


>>> to just change which mirror we read to based on
>>> some arbitrary user policy.  I assume this is to solve the case where you have
>>> slow and fast disks, so you can always read from the fast disk?  And then it's
>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
>>> particular obscure setup.  Thanks,
>>>
>>> Josef
>>
>> Not commenting on the code itself, but as a user I see this SSD RAID1
>> acceleration as a future much have feature. It's only obscure at the
>> moment because we don't have code to take advantage of it. But on
>> large btrfs filesystems with hundreds of GB of metadata, like I have
>> for backups, the usability of the filesystem is dramatically improved
>> having the metadata on an SSD( though currently only half of the time
>> due to the even/odd pid distribution.)
> 
> But that's different from a mirror.  100% it would be nice to say "put my
> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
> patch is specifically about changing which drive we choose in a mirrored setup,
> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
> but we can accomplish that by just adding a check for nonrotational in the code,
> and then we don't have to encode all this nonsense in the file system.  Thanks,

  I wrote about the readmirror policy framework here[2],
  I forgot to link it here, sorry about that, my mistake.

  [2]
 
https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/

  Readmirror policy is for raid1, raid10 and future N way mirror.
  Yes for now its only for raid1.

  Here the idea is to create a framework so that readmirror policy
  can be configured as needed. And nonrotational can be one such policy.

  The example of hard-coded nonrotational policy does not work in case
  of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
  device, as all these are still nonrotational devices. So hard-coded
  policy is not a good idea. If we have to hardcode then there is Q-depth
  based readmirror routing is better (patch in the ML), but that is
  not good enough, because some configs wants it based on the disk-LBA
  so that SAN storage target cache is balanced and not duplicated.
  So in short it must be a configurable policy.

  devid policy is the first policy which is for the advance users when
  they know what they are doing, which is sure to support any types
  of HW configurations/combinations (except for the cache balance).

  So in total potential configurable policies are:

  - pid     - original. dropped as a policy because of the comments
              received [3].
  - Q-depth - patches are in the ML this can be the default policy.
  - LBA     - to avoid duplicating the cache on the storage target
              in SAN
  - devid   - as discussed above.
  - nonrotational - as discussed above.

  So there are 5 ways to configure as needed, so a framework
  infrastructure is worth?

[3]
https://lore.kernel.org/linux-btrfs/20190409154840.GM29086@twin.jikos.cz/

Thanks, Anand

> Josef
>
Anand Jain Sept. 12, 2019, 7:48 a.m. UTC | #6
On 12/9/19 12:37 AM, David Sterba wrote:
> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
>> thread pid to determine the %mirror_num when the mirror_num=0.
>>
>> This patch introduces a framework so that we can add policies to determine
>> the %mirror_num. And also adds the devid as the readmirror policy.
>>
>> The new property is stored as an item in the device tree as show below.
>>      (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
>>
>> To be able to set and get this new property also introduces new ioctls
>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
>> is defined as
>>          struct btrfs_ioctl_readmirror_args {
>>                  __u64 type; /* RW */
>>                  __u64 device_bitmap; /* RW */
>>          }
> 
> I don't remember if there was a suggestion to use ioctls for read
> mirror, but the property interface should be sufficient. Besides this
> ioctl interafce is quite an anti-pattern: narrow use, non-extensible
> structure, alternative and more convenient interface already available.

  Extended attribute interface f(get/set)attr is inode bound, but
  the readmirror property is filesystem bound. For the readmirror we
  can still use the extended attribute, but it might be considered as
  abuse which we haven't done so far, here below [1] is the list of
  property with the interface it uses and where the property is saved.

[1]
  property      interface  save-as
  ro            ioctl      root-item
  label         ioctl      super-block
  compression   xattr      xattr
  v1:readmirror xattr      xattr
  v2:readmirror ioctl      dev-tree-item

  You are asking for the combination of
    property   interface  save-as
    readmirror xattr      dev-tree-item

  I can give a try.

>> An usage example as follows:
>>          btrfs property set /btrfs readmirror devid:1,3
>>          btrfs property get /btrfs readmirror
>>            readmirror devid:1 3
>>          btrfs property set /btrfs readmirror ""
>>          btrfs property get /btrfs readmirror
>>            readmirror default
>>
>> This patchset has been tested completely, however marked as RFC for the
>> following reasons and comments on them (or any other) are appreciated as
>> usual.
>>   . The new objectid is defined as
>>        #define BTRFS_READMIRROR_OBJECTID -1ULL
>>     Need consent we are fine to use this value, and with this value it
>>     shall be placed just before the DEV_STATS_OBJECTID item which is more
>>     frequently used only during the device errors.
> 
> -1 can be considered a special value in other cases, not necessarily
> here but if the ordering of items is the only reason I'd say no. The
> keys/items will most likely land in the same node so there's no point
> forcing the order.

  Agreed. Any suggestion on the value for the BTRFS_READMIRROR_OBJECTID.

>> .  I am using a u64 bitmap to represent the devices id, so the max device
>>     id that we could represent is 63, its a kind of limitation which should
>>     be addressed before integration, I wonder if there is any suggestion?
> 
> Uh 63, so now an implementation detail is posing a global limit? That
> sounds backwards.

   Yes. And I was thinking of u64 array but that doesn't scale as well.
   Anyways I have in the list to try using xattr interface which may
   address this issue.

>>     Kindly note that, multiple ioctls with each time representing a set of
>>     device(s) is not a choice because we need to make sure the readmirror
>>     changes happens in a commit transaction.
> 
> I believe this can be guaranteed by the properties interface, ie. single
> value gets processed at once and with some locking around the state of
> devices can be updated atomically.
> 
> The open question is still how to store the readmirror property
> per-device, it could be either an item or bit inside the device
> structure.

  We discussed that before here.

 
https://lore.kernel.org/linux-btrfs/8d31c3a2-3fb0-63af-3173-948ed0e84de3@oracle.com/

> Besides the interface, I'm kind of missing the usecase description what
> is expected from the read mirror policies and how they could affect
> various scenarios. Maybe it was in some of the previous iterations, it's
> hard too track everything so this should be part of the cover letter (or
> at leat linked if it's too much text).
> 

  Yep. My mistake I missed it link it. Sorry about that.

Thanks, Anand
Josef Bacik Sept. 12, 2019, 9:50 a.m. UTC | #7
On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
> 
> 
> Thanks for the comments. More below.
> 
> On 12/9/19 3:16 AM, Josef Bacik wrote:
> > On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
> > > On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
> > > > 
> > > > On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> > > > > Function call chain  __btrfs_map_block()->find_live_mirror() uses
> > > > > thread pid to determine the %mirror_num when the mirror_num=0.
> > > > > 
> > > > > This patch introduces a framework so that we can add policies to determine
> > > > > the %mirror_num. And also adds the devid as the readmirror policy.
> > > > > 
> > > > > The new property is stored as an item in the device tree as show below.
> > > > >      (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> > > > > 
> > > > > To be able to set and get this new property also introduces new ioctls
> > > > > BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> > > > > is defined as
> > > > >          struct btrfs_ioctl_readmirror_args {
> > > > >                  __u64 type; /* RW */
> > > > >                  __u64 device_bitmap; /* RW */
> > > > >          }
> > > > > 
> > > > > An usage example as follows:
> > > > >          btrfs property set /btrfs readmirror devid:1,3
> > > > >          btrfs property get /btrfs readmirror
> > > > >            readmirror devid:1 3
> > > > >          btrfs property set /btrfs readmirror ""
> > > > >          btrfs property get /btrfs readmirror
> > > > >            readmirror default
> > > > > 
> > > > > This patchset has been tested completely, however marked as RFC for the
> > > > > following reasons and comments on them (or any other) are appreciated as
> > > > > usual.
> > > > >   . The new objectid is defined as
> > > > >        #define BTRFS_READMIRROR_OBJECTID -1ULL
> > > > >     Need consent we are fine to use this value, and with this value it
> > > > >     shall be placed just before the DEV_STATS_OBJECTID item which is more
> > > > >     frequently used only during the device errors.
> > > > > 
> > > > > .  I am using a u64 bitmap to represent the devices id, so the max device
> > > > >     id that we could represent is 63, its a kind of limitation which should
> > > > >     be addressed before integration, I wonder if there is any suggestion?
> > > > >     Kindly note that, multiple ioctls with each time representing a set of
> > > > >     device(s) is not a choice because we need to make sure the readmirror
> > > > >     changes happens in a commit transaction.
> > > > > 
> > > > > v1->RFC v2:
> > > > >    . Property is stored as a dev-tree item instead of root inode extended
> > > > >      attribute.
> > > > >    . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> > > > >    . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> > > > > 
> > > > > RFC->v1:
> > > > >    Drops pid as one of the readmirror policy choices and as usual remains
> > > > >    as default. And when the devid is reset the readmirror policy falls back
> > > > >    to pid.
> > > > >    Drops the mount -o readmirror idea, it can be added at a later point of
> > > > >    time.
> > > > >    Property now accepts more than 1 devid as readmirror device. As shown
> > > > >    in the example above.
> > > > > 
> > > > 
> > > > This is a lot of infrastructure
> 
>   Ok. Any idea on a better implementation?
>   How about extended attribute approach? v1 patches proposed
>   it, but it abused the extended attribute as commented here [1]
>   and v2 got changed to an item-key.
> 
> [1]
> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
>

That's a NAK on the prop interface.  This is a fs wide policy, not a
directory/inode policy.
 
> 
> > > > to just change which mirror we read to based on
> > > > some arbitrary user policy.  I assume this is to solve the case where you have
> > > > slow and fast disks, so you can always read from the fast disk?  And then it's
> > > > only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> > > > SSD and a normal disk?  I'm not seeing a point to this much code for one
> > > > particular obscure setup.  Thanks,
> > > > 
> > > > Josef
> > > 
> > > Not commenting on the code itself, but as a user I see this SSD RAID1
> > > acceleration as a future much have feature. It's only obscure at the
> > > moment because we don't have code to take advantage of it. But on
> > > large btrfs filesystems with hundreds of GB of metadata, like I have
> > > for backups, the usability of the filesystem is dramatically improved
> > > having the metadata on an SSD( though currently only half of the time
> > > due to the even/odd pid distribution.)
> > 
> > But that's different from a mirror.  100% it would be nice to say "put my
> > metadata on the ssd, data elsewhere".  That's not what this patch is about, this
> > patch is specifically about changing which drive we choose in a mirrored setup,
> > which is super unlikely to mirror a SSD with a slow drive, cause it's just going
> > to be slow no matter what.  Sure we could make it so reads always go to the SSD,
> > but we can accomplish that by just adding a check for nonrotational in the code,
> > and then we don't have to encode all this nonsense in the file system.  Thanks,
> 
>  I wrote about the readmirror policy framework here[2],
>  I forgot to link it here, sorry about that, my mistake.
> 
>  [2]
> 
> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
> 
>  Readmirror policy is for raid1, raid10 and future N way mirror.
>  Yes for now its only for raid1.
> 
>  Here the idea is to create a framework so that readmirror policy
>  can be configured as needed. And nonrotational can be one such policy.
> 
>  The example of hard-coded nonrotational policy does not work in case
>  of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
>  device, as all these are still nonrotational devices. So hard-coded
>  policy is not a good idea. If we have to hardcode then there is Q-depth
>  based readmirror routing is better (patch in the ML), but that is
>  not good enough, because some configs wants it based on the disk-LBA
>  so that SAN storage target cache is balanced and not duplicated.
>  So in short it must be a configurable policy.
> 

Again, if you are mixing disk types you likely always want non-rotational, but
still mixing different speed devices in a mirror setup is just asking for weird
latency problems.  I don't think solving this use case is necessary.  If you mix
ssd + network device in a serious production setup then you probably should be
fired cause you don't know what you are doing.  Having the generic
"nonrotational gets priority" is going to cover 99% of the actual use cases that
make sense.

The SAN usecase I can sort of see, but again I don't feel like it's a problem we
need to solve with on-disk format.  Add a priority to sysfs so you can change it
with udev or something on the fly.  Thanks,

Josef
Anand Jain Sept. 12, 2019, 10 a.m. UTC | #8
> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> 
> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
>> 
>> 
>> Thanks for the comments. More below.
>> 
>> On 12/9/19 3:16 AM, Josef Bacik wrote:
>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>>>> 
>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
>>>>>> 
>>>>>> This patch introduces a framework so that we can add policies to determine
>>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
>>>>>> 
>>>>>> The new property is stored as an item in the device tree as show below.
>>>>>>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
>>>>>> 
>>>>>> To be able to set and get this new property also introduces new ioctls
>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
>>>>>> is defined as
>>>>>>         struct btrfs_ioctl_readmirror_args {
>>>>>>                 __u64 type; /* RW */
>>>>>>                 __u64 device_bitmap; /* RW */
>>>>>>         }
>>>>>> 
>>>>>> An usage example as follows:
>>>>>>         btrfs property set /btrfs readmirror devid:1,3
>>>>>>         btrfs property get /btrfs readmirror
>>>>>>           readmirror devid:1 3
>>>>>>         btrfs property set /btrfs readmirror ""
>>>>>>         btrfs property get /btrfs readmirror
>>>>>>           readmirror default
>>>>>> 
>>>>>> This patchset has been tested completely, however marked as RFC for the
>>>>>> following reasons and comments on them (or any other) are appreciated as
>>>>>> usual.
>>>>>>  . The new objectid is defined as
>>>>>>       #define BTRFS_READMIRROR_OBJECTID -1ULL
>>>>>>    Need consent we are fine to use this value, and with this value it
>>>>>>    shall be placed just before the DEV_STATS_OBJECTID item which is more
>>>>>>    frequently used only during the device errors.
>>>>>> 
>>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
>>>>>>    id that we could represent is 63, its a kind of limitation which should
>>>>>>    be addressed before integration, I wonder if there is any suggestion?
>>>>>>    Kindly note that, multiple ioctls with each time representing a set of
>>>>>>    device(s) is not a choice because we need to make sure the readmirror
>>>>>>    changes happens in a commit transaction.
>>>>>> 
>>>>>> v1->RFC v2:
>>>>>>   . Property is stored as a dev-tree item instead of root inode extended
>>>>>>     attribute.
>>>>>>   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
>>>>>>   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
>>>>>> 
>>>>>> RFC->v1:
>>>>>>   Drops pid as one of the readmirror policy choices and as usual remains
>>>>>>   as default. And when the devid is reset the readmirror policy falls back
>>>>>>   to pid.
>>>>>>   Drops the mount -o readmirror idea, it can be added at a later point of
>>>>>>   time.
>>>>>>   Property now accepts more than 1 devid as readmirror device. As shown
>>>>>>   in the example above.
>>>>>> 
>>>>> 
>>>>> This is a lot of infrastructure
>> 
>>  Ok. Any idea on a better implementation?
>>  How about extended attribute approach? v1 patches proposed
>>  it, but it abused the extended attribute as commented here [1]
>>  and v2 got changed to an item-key.
>> 
>> [1]
>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
>> 
> 
> That's a NAK on the prop interface.  This is a fs wide policy, not a
> directory/inode policy.
> 
>> 
>>>>> to just change which mirror we read to based on
>>>>> some arbitrary user policy.  I assume this is to solve the case where you have
>>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
>>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
>>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
>>>>> particular obscure setup.  Thanks,
>>>>> 
>>>>> Josef
>>>> 
>>>> Not commenting on the code itself, but as a user I see this SSD RAID1
>>>> acceleration as a future much have feature. It's only obscure at the
>>>> moment because we don't have code to take advantage of it. But on
>>>> large btrfs filesystems with hundreds of GB of metadata, like I have
>>>> for backups, the usability of the filesystem is dramatically improved
>>>> having the metadata on an SSD( though currently only half of the time
>>>> due to the even/odd pid distribution.)
>>> 
>>> But that's different from a mirror.  100% it would be nice to say "put my
>>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
>>> patch is specifically about changing which drive we choose in a mirrored setup,
>>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
>>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
>>> but we can accomplish that by just adding a check for nonrotational in the code,
>>> and then we don't have to encode all this nonsense in the file system.  Thanks,
>> 
>> I wrote about the readmirror policy framework here[2],
>> I forgot to link it here, sorry about that, my mistake.
>> 
>> [2]
>> 
>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
>> 
>> Readmirror policy is for raid1, raid10 and future N way mirror.
>> Yes for now its only for raid1.
>> 
>> Here the idea is to create a framework so that readmirror policy
>> can be configured as needed. And nonrotational can be one such policy.
>> 
>> The example of hard-coded nonrotational policy does not work in case
>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
>> device, as all these are still nonrotational devices. So hard-coded
>> policy is not a good idea. If we have to hardcode then there is Q-depth
>> based readmirror routing is better (patch in the ML), but that is
>> not good enough, because some configs wants it based on the disk-LBA
>> so that SAN storage target cache is balanced and not duplicated.
>> So in short it must be a configurable policy.
>> 
> 
> Again, if you are mixing disk types you likely always want non-rotational, but
> still mixing different speed devices in a mirror setup is just asking for weird
> latency problems.  I don't think solving this use case is necessary.  If you mix
> ssd + network device in a serious production setup then you probably should be
> fired cause you don't know what you are doing.  Having the generic
> "nonrotational gets priority" is going to cover 99% of the actual use cases that
> make sense.
> 
> The SAN usecase I can sort of see, but again I don't feel like it's a problem we
> need to solve with on-disk format.  Add a priority to sysfs so you can change it
> with udev or something on the fly.  Thanks,
> 
 
 Ok.
 Sysfs is fine however we need configuration to be persistent across reboots.
 Any idea?

Thanks, Anand

> Josef
Josef Bacik Sept. 12, 2019, 10:03 a.m. UTC | #9
On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
> 
> 
> > On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> > 
> > On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
> >> 
> >> 
> >> Thanks for the comments. More below.
> >> 
> >> On 12/9/19 3:16 AM, Josef Bacik wrote:
> >>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
> >>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
> >>>>> 
> >>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> >>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
> >>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
> >>>>>> 
> >>>>>> This patch introduces a framework so that we can add policies to determine
> >>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
> >>>>>> 
> >>>>>> The new property is stored as an item in the device tree as show below.
> >>>>>>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> >>>>>> 
> >>>>>> To be able to set and get this new property also introduces new ioctls
> >>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> >>>>>> is defined as
> >>>>>>         struct btrfs_ioctl_readmirror_args {
> >>>>>>                 __u64 type; /* RW */
> >>>>>>                 __u64 device_bitmap; /* RW */
> >>>>>>         }
> >>>>>> 
> >>>>>> An usage example as follows:
> >>>>>>         btrfs property set /btrfs readmirror devid:1,3
> >>>>>>         btrfs property get /btrfs readmirror
> >>>>>>           readmirror devid:1 3
> >>>>>>         btrfs property set /btrfs readmirror ""
> >>>>>>         btrfs property get /btrfs readmirror
> >>>>>>           readmirror default
> >>>>>> 
> >>>>>> This patchset has been tested completely, however marked as RFC for the
> >>>>>> following reasons and comments on them (or any other) are appreciated as
> >>>>>> usual.
> >>>>>>  . The new objectid is defined as
> >>>>>>       #define BTRFS_READMIRROR_OBJECTID -1ULL
> >>>>>>    Need consent we are fine to use this value, and with this value it
> >>>>>>    shall be placed just before the DEV_STATS_OBJECTID item which is more
> >>>>>>    frequently used only during the device errors.
> >>>>>> 
> >>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
> >>>>>>    id that we could represent is 63, its a kind of limitation which should
> >>>>>>    be addressed before integration, I wonder if there is any suggestion?
> >>>>>>    Kindly note that, multiple ioctls with each time representing a set of
> >>>>>>    device(s) is not a choice because we need to make sure the readmirror
> >>>>>>    changes happens in a commit transaction.
> >>>>>> 
> >>>>>> v1->RFC v2:
> >>>>>>   . Property is stored as a dev-tree item instead of root inode extended
> >>>>>>     attribute.
> >>>>>>   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> >>>>>>   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> >>>>>> 
> >>>>>> RFC->v1:
> >>>>>>   Drops pid as one of the readmirror policy choices and as usual remains
> >>>>>>   as default. And when the devid is reset the readmirror policy falls back
> >>>>>>   to pid.
> >>>>>>   Drops the mount -o readmirror idea, it can be added at a later point of
> >>>>>>   time.
> >>>>>>   Property now accepts more than 1 devid as readmirror device. As shown
> >>>>>>   in the example above.
> >>>>>> 
> >>>>> 
> >>>>> This is a lot of infrastructure
> >> 
> >>  Ok. Any idea on a better implementation?
> >>  How about extended attribute approach? v1 patches proposed
> >>  it, but it abused the extended attribute as commented here [1]
> >>  and v2 got changed to an item-key.
> >> 
> >> [1]
> >> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
> >> 
> > 
> > That's a NAK on the prop interface.  This is a fs wide policy, not a
> > directory/inode policy.
> > 
> >> 
> >>>>> to just change which mirror we read to based on
> >>>>> some arbitrary user policy.  I assume this is to solve the case where you have
> >>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
> >>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> >>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
> >>>>> particular obscure setup.  Thanks,
> >>>>> 
> >>>>> Josef
> >>>> 
> >>>> Not commenting on the code itself, but as a user I see this SSD RAID1
> >>>> acceleration as a future much have feature. It's only obscure at the
> >>>> moment because we don't have code to take advantage of it. But on
> >>>> large btrfs filesystems with hundreds of GB of metadata, like I have
> >>>> for backups, the usability of the filesystem is dramatically improved
> >>>> having the metadata on an SSD( though currently only half of the time
> >>>> due to the even/odd pid distribution.)
> >>> 
> >>> But that's different from a mirror.  100% it would be nice to say "put my
> >>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
> >>> patch is specifically about changing which drive we choose in a mirrored setup,
> >>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
> >>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
> >>> but we can accomplish that by just adding a check for nonrotational in the code,
> >>> and then we don't have to encode all this nonsense in the file system.  Thanks,
> >> 
> >> I wrote about the readmirror policy framework here[2],
> >> I forgot to link it here, sorry about that, my mistake.
> >> 
> >> [2]
> >> 
> >> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
> >> 
> >> Readmirror policy is for raid1, raid10 and future N way mirror.
> >> Yes for now its only for raid1.
> >> 
> >> Here the idea is to create a framework so that readmirror policy
> >> can be configured as needed. And nonrotational can be one such policy.
> >> 
> >> The example of hard-coded nonrotational policy does not work in case
> >> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
> >> device, as all these are still nonrotational devices. So hard-coded
> >> policy is not a good idea. If we have to hardcode then there is Q-depth
> >> based readmirror routing is better (patch in the ML), but that is
> >> not good enough, because some configs wants it based on the disk-LBA
> >> so that SAN storage target cache is balanced and not duplicated.
> >> So in short it must be a configurable policy.
> >> 
> > 
> > Again, if you are mixing disk types you likely always want non-rotational, but
> > still mixing different speed devices in a mirror setup is just asking for weird
> > latency problems.  I don't think solving this use case is necessary.  If you mix
> > ssd + network device in a serious production setup then you probably should be
> > fired cause you don't know what you are doing.  Having the generic
> > "nonrotational gets priority" is going to cover 99% of the actual use cases that
> > make sense.
> > 
> > The SAN usecase I can sort of see, but again I don't feel like it's a problem we
> > need to solve with on-disk format.  Add a priority to sysfs so you can change it
> > with udev or something on the fly.  Thanks,
> > 
>  
>  Ok.
>  Sysfs is fine however we need configuration to be persistent across reboots.
>  Any idea?
>

Udev rules.  Thanks,

Josef
Anand Jain Sept. 12, 2019, 10:10 a.m. UTC | #10
> On 12 Sep 2019, at 6:03 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> 
> On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
>> 
>> 
>>> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
>>> 
>>> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
>>>> 
>>>> 
>>>> Thanks for the comments. More below.
>>>> 
>>>> On 12/9/19 3:16 AM, Josef Bacik wrote:
>>>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
>>>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>>>>>> 
>>>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>>>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
>>>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
>>>>>>>> 
>>>>>>>> This patch introduces a framework so that we can add policies to determine
>>>>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
>>>>>>>> 
>>>>>>>> The new property is stored as an item in the device tree as show below.
>>>>>>>>    (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
>>>>>>>> 
>>>>>>>> To be able to set and get this new property also introduces new ioctls
>>>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
>>>>>>>> is defined as
>>>>>>>>        struct btrfs_ioctl_readmirror_args {
>>>>>>>>                __u64 type; /* RW */
>>>>>>>>                __u64 device_bitmap; /* RW */
>>>>>>>>        }
>>>>>>>> 
>>>>>>>> An usage example as follows:
>>>>>>>>        btrfs property set /btrfs readmirror devid:1,3
>>>>>>>>        btrfs property get /btrfs readmirror
>>>>>>>>          readmirror devid:1 3
>>>>>>>>        btrfs property set /btrfs readmirror ""
>>>>>>>>        btrfs property get /btrfs readmirror
>>>>>>>>          readmirror default
>>>>>>>> 
>>>>>>>> This patchset has been tested completely, however marked as RFC for the
>>>>>>>> following reasons and comments on them (or any other) are appreciated as
>>>>>>>> usual.
>>>>>>>> . The new objectid is defined as
>>>>>>>>      #define BTRFS_READMIRROR_OBJECTID -1ULL
>>>>>>>>   Need consent we are fine to use this value, and with this value it
>>>>>>>>   shall be placed just before the DEV_STATS_OBJECTID item which is more
>>>>>>>>   frequently used only during the device errors.
>>>>>>>> 
>>>>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
>>>>>>>>   id that we could represent is 63, its a kind of limitation which should
>>>>>>>>   be addressed before integration, I wonder if there is any suggestion?
>>>>>>>>   Kindly note that, multiple ioctls with each time representing a set of
>>>>>>>>   device(s) is not a choice because we need to make sure the readmirror
>>>>>>>>   changes happens in a commit transaction.
>>>>>>>> 
>>>>>>>> v1->RFC v2:
>>>>>>>>  . Property is stored as a dev-tree item instead of root inode extended
>>>>>>>>    attribute.
>>>>>>>>  . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
>>>>>>>>  . Changed format specifier from devid1,2,3.. to devid:1,2,3..
>>>>>>>> 
>>>>>>>> RFC->v1:
>>>>>>>>  Drops pid as one of the readmirror policy choices and as usual remains
>>>>>>>>  as default. And when the devid is reset the readmirror policy falls back
>>>>>>>>  to pid.
>>>>>>>>  Drops the mount -o readmirror idea, it can be added at a later point of
>>>>>>>>  time.
>>>>>>>>  Property now accepts more than 1 devid as readmirror device. As shown
>>>>>>>>  in the example above.
>>>>>>>> 
>>>>>>> 
>>>>>>> This is a lot of infrastructure
>>>> 
>>>> Ok. Any idea on a better implementation?
>>>> How about extended attribute approach? v1 patches proposed
>>>> it, but it abused the extended attribute as commented here [1]
>>>> and v2 got changed to an item-key.
>>>> 
>>>> [1]
>>>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
>>>> 
>>> 
>>> That's a NAK on the prop interface.  This is a fs wide policy, not a
>>> directory/inode policy.
>>> 
>>>> 
>>>>>>> to just change which mirror we read to based on
>>>>>>> some arbitrary user policy.  I assume this is to solve the case where you have
>>>>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
>>>>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
>>>>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
>>>>>>> particular obscure setup.  Thanks,
>>>>>>> 
>>>>>>> Josef
>>>>>> 
>>>>>> Not commenting on the code itself, but as a user I see this SSD RAID1
>>>>>> acceleration as a future much have feature. It's only obscure at the
>>>>>> moment because we don't have code to take advantage of it. But on
>>>>>> large btrfs filesystems with hundreds of GB of metadata, like I have
>>>>>> for backups, the usability of the filesystem is dramatically improved
>>>>>> having the metadata on an SSD( though currently only half of the time
>>>>>> due to the even/odd pid distribution.)
>>>>> 
>>>>> But that's different from a mirror.  100% it would be nice to say "put my
>>>>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
>>>>> patch is specifically about changing which drive we choose in a mirrored setup,
>>>>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
>>>>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
>>>>> but we can accomplish that by just adding a check for nonrotational in the code,
>>>>> and then we don't have to encode all this nonsense in the file system.  Thanks,
>>>> 
>>>> I wrote about the readmirror policy framework here[2],
>>>> I forgot to link it here, sorry about that, my mistake.
>>>> 
>>>> [2]
>>>> 
>>>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
>>>> 
>>>> Readmirror policy is for raid1, raid10 and future N way mirror.
>>>> Yes for now its only for raid1.
>>>> 
>>>> Here the idea is to create a framework so that readmirror policy
>>>> can be configured as needed. And nonrotational can be one such policy.
>>>> 
>>>> The example of hard-coded nonrotational policy does not work in case
>>>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
>>>> device, as all these are still nonrotational devices. So hard-coded
>>>> policy is not a good idea. If we have to hardcode then there is Q-depth
>>>> based readmirror routing is better (patch in the ML), but that is
>>>> not good enough, because some configs wants it based on the disk-LBA
>>>> so that SAN storage target cache is balanced and not duplicated.
>>>> So in short it must be a configurable policy.
>>>> 
>>> 
>>> Again, if you are mixing disk types you likely always want non-rotational, but
>>> still mixing different speed devices in a mirror setup is just asking for weird
>>> latency problems.  I don't think solving this use case is necessary.  If you mix
>>> ssd + network device in a serious production setup then you probably should be
>>> fired cause you don't know what you are doing.  Having the generic
>>> "nonrotational gets priority" is going to cover 99% of the actual use cases that
>>> make sense.
>>> 
>>> The SAN usecase I can sort of see, but again I don't feel like it's a problem we
>>> need to solve with on-disk format.  Add a priority to sysfs so you can change it
>>> with udev or something on the fly.  Thanks,
>>> 
>> 
>> Ok.
>> Sysfs is fine however we need configuration to be persistent across reboots.
>> Any idea?
>> 
> 
> Udev rules.  Thanks,
> 

 Josef, configs moving along with the luns in san seems to be more
 easy to manage, otherwise when the host fails, each potential new
 server has to be pre-configured with the udev rules. 

Thanks, Anand

> Josef
Josef Bacik Sept. 12, 2019, 10:13 a.m. UTC | #11
On Thu, Sep 12, 2019 at 06:10:08PM +0800, Anand Jain wrote:
> 
> 
> > On 12 Sep 2019, at 6:03 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> > 
> > On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
> >> 
> >> 
> >>> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> >>> 
> >>> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
> >>>> 
> >>>> 
> >>>> Thanks for the comments. More below.
> >>>> 
> >>>> On 12/9/19 3:16 AM, Josef Bacik wrote:
> >>>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
> >>>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
> >>>>>>> 
> >>>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> >>>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
> >>>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
> >>>>>>>> 
> >>>>>>>> This patch introduces a framework so that we can add policies to determine
> >>>>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
> >>>>>>>> 
> >>>>>>>> The new property is stored as an item in the device tree as show below.
> >>>>>>>>    (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> >>>>>>>> 
> >>>>>>>> To be able to set and get this new property also introduces new ioctls
> >>>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> >>>>>>>> is defined as
> >>>>>>>>        struct btrfs_ioctl_readmirror_args {
> >>>>>>>>                __u64 type; /* RW */
> >>>>>>>>                __u64 device_bitmap; /* RW */
> >>>>>>>>        }
> >>>>>>>> 
> >>>>>>>> An usage example as follows:
> >>>>>>>>        btrfs property set /btrfs readmirror devid:1,3
> >>>>>>>>        btrfs property get /btrfs readmirror
> >>>>>>>>          readmirror devid:1 3
> >>>>>>>>        btrfs property set /btrfs readmirror ""
> >>>>>>>>        btrfs property get /btrfs readmirror
> >>>>>>>>          readmirror default
> >>>>>>>> 
> >>>>>>>> This patchset has been tested completely, however marked as RFC for the
> >>>>>>>> following reasons and comments on them (or any other) are appreciated as
> >>>>>>>> usual.
> >>>>>>>> . The new objectid is defined as
> >>>>>>>>      #define BTRFS_READMIRROR_OBJECTID -1ULL
> >>>>>>>>   Need consent we are fine to use this value, and with this value it
> >>>>>>>>   shall be placed just before the DEV_STATS_OBJECTID item which is more
> >>>>>>>>   frequently used only during the device errors.
> >>>>>>>> 
> >>>>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
> >>>>>>>>   id that we could represent is 63, its a kind of limitation which should
> >>>>>>>>   be addressed before integration, I wonder if there is any suggestion?
> >>>>>>>>   Kindly note that, multiple ioctls with each time representing a set of
> >>>>>>>>   device(s) is not a choice because we need to make sure the readmirror
> >>>>>>>>   changes happens in a commit transaction.
> >>>>>>>> 
> >>>>>>>> v1->RFC v2:
> >>>>>>>>  . Property is stored as a dev-tree item instead of root inode extended
> >>>>>>>>    attribute.
> >>>>>>>>  . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> >>>>>>>>  . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> >>>>>>>> 
> >>>>>>>> RFC->v1:
> >>>>>>>>  Drops pid as one of the readmirror policy choices and as usual remains
> >>>>>>>>  as default. And when the devid is reset the readmirror policy falls back
> >>>>>>>>  to pid.
> >>>>>>>>  Drops the mount -o readmirror idea, it can be added at a later point of
> >>>>>>>>  time.
> >>>>>>>>  Property now accepts more than 1 devid as readmirror device. As shown
> >>>>>>>>  in the example above.
> >>>>>>>> 
> >>>>>>> 
> >>>>>>> This is a lot of infrastructure
> >>>> 
> >>>> Ok. Any idea on a better implementation?
> >>>> How about extended attribute approach? v1 patches proposed
> >>>> it, but it abused the extended attribute as commented here [1]
> >>>> and v2 got changed to an item-key.
> >>>> 
> >>>> [1]
> >>>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
> >>>> 
> >>> 
> >>> That's a NAK on the prop interface.  This is a fs wide policy, not a
> >>> directory/inode policy.
> >>> 
> >>>> 
> >>>>>>> to just change which mirror we read to based on
> >>>>>>> some arbitrary user policy.  I assume this is to solve the case where you have
> >>>>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
> >>>>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> >>>>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
> >>>>>>> particular obscure setup.  Thanks,
> >>>>>>> 
> >>>>>>> Josef
> >>>>>> 
> >>>>>> Not commenting on the code itself, but as a user I see this SSD RAID1
> >>>>>> acceleration as a future much have feature. It's only obscure at the
> >>>>>> moment because we don't have code to take advantage of it. But on
> >>>>>> large btrfs filesystems with hundreds of GB of metadata, like I have
> >>>>>> for backups, the usability of the filesystem is dramatically improved
> >>>>>> having the metadata on an SSD( though currently only half of the time
> >>>>>> due to the even/odd pid distribution.)
> >>>>> 
> >>>>> But that's different from a mirror.  100% it would be nice to say "put my
> >>>>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
> >>>>> patch is specifically about changing which drive we choose in a mirrored setup,
> >>>>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
> >>>>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
> >>>>> but we can accomplish that by just adding a check for nonrotational in the code,
> >>>>> and then we don't have to encode all this nonsense in the file system.  Thanks,
> >>>> 
> >>>> I wrote about the readmirror policy framework here[2],
> >>>> I forgot to link it here, sorry about that, my mistake.
> >>>> 
> >>>> [2]
> >>>> 
> >>>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
> >>>> 
> >>>> Readmirror policy is for raid1, raid10 and future N way mirror.
> >>>> Yes for now its only for raid1.
> >>>> 
> >>>> Here the idea is to create a framework so that readmirror policy
> >>>> can be configured as needed. And nonrotational can be one such policy.
> >>>> 
> >>>> The example of hard-coded nonrotational policy does not work in case
> >>>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
> >>>> device, as all these are still nonrotational devices. So hard-coded
> >>>> policy is not a good idea. If we have to hardcode then there is Q-depth
> >>>> based readmirror routing is better (patch in the ML), but that is
> >>>> not good enough, because some configs wants it based on the disk-LBA
> >>>> so that SAN storage target cache is balanced and not duplicated.
> >>>> So in short it must be a configurable policy.
> >>>> 
> >>> 
> >>> Again, if you are mixing disk types you likely always want non-rotational, but
> >>> still mixing different speed devices in a mirror setup is just asking for weird
> >>> latency problems.  I don't think solving this use case is necessary.  If you mix
> >>> ssd + network device in a serious production setup then you probably should be
> >>> fired cause you don't know what you are doing.  Having the generic
> >>> "nonrotational gets priority" is going to cover 99% of the actual use cases that
> >>> make sense.
> >>> 
> >>> The SAN usecase I can sort of see, but again I don't feel like it's a problem we
> >>> need to solve with on-disk format.  Add a priority to sysfs so you can change it
> >>> with udev or something on the fly.  Thanks,
> >>> 
> >> 
> >> Ok.
> >> Sysfs is fine however we need configuration to be persistent across reboots.
> >> Any idea?
> >> 
> > 
> > Udev rules.  Thanks,
> > 
> 
>  Josef, configs moving along with the luns in san seems to be more
>  easy to manage, otherwise when the host fails, each potential new
>  server has to be pre-configured with the udev rules. 
> 

It's 2019, if people haven't figured out how to do persistent configuration by
now then all hope is lost.  Facebook persistently configures millions of
machines, I'm sure people can figure out how to make sure a udev rule ends up on
the right host connected to a SAN that doesn't move.  Thanks,

Josef
Anand Jain Sept. 16, 2019, 8:19 a.m. UTC | #12
On 12/9/19 6:13 PM, Josef Bacik wrote:
> On Thu, Sep 12, 2019 at 06:10:08PM +0800, Anand Jain wrote:
>>
>>
>>> On 12 Sep 2019, at 6:03 PM, Josef Bacik <josef@toxicpanda.com> wrote:
>>>
>>> On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
>>>>
>>>>
>>>>> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
>>>>>
>>>>> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
>>>>>>
>>>>>>
>>>>>> Thanks for the comments. More below.
>>>>>>
>>>>>> On 12/9/19 3:16 AM, Josef Bacik wrote:
>>>>>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
>>>>>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
>>>>>>>>>
>>>>>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>>>>>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
>>>>>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
>>>>>>>>>>
>>>>>>>>>> This patch introduces a framework so that we can add policies to determine
>>>>>>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
>>>>>>>>>>
>>>>>>>>>> The new property is stored as an item in the device tree as show below.
>>>>>>>>>>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
>>>>>>>>>>
>>>>>>>>>> To be able to set and get this new property also introduces new ioctls
>>>>>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
>>>>>>>>>> is defined as
>>>>>>>>>>         struct btrfs_ioctl_readmirror_args {
>>>>>>>>>>                 __u64 type; /* RW */
>>>>>>>>>>                 __u64 device_bitmap; /* RW */
>>>>>>>>>>         }
>>>>>>>>>>
>>>>>>>>>> An usage example as follows:
>>>>>>>>>>         btrfs property set /btrfs readmirror devid:1,3
>>>>>>>>>>         btrfs property get /btrfs readmirror
>>>>>>>>>>           readmirror devid:1 3
>>>>>>>>>>         btrfs property set /btrfs readmirror ""
>>>>>>>>>>         btrfs property get /btrfs readmirror
>>>>>>>>>>           readmirror default
>>>>>>>>>>
>>>>>>>>>> This patchset has been tested completely, however marked as RFC for the
>>>>>>>>>> following reasons and comments on them (or any other) are appreciated as
>>>>>>>>>> usual.
>>>>>>>>>> . The new objectid is defined as
>>>>>>>>>>       #define BTRFS_READMIRROR_OBJECTID -1ULL
>>>>>>>>>>    Need consent we are fine to use this value, and with this value it
>>>>>>>>>>    shall be placed just before the DEV_STATS_OBJECTID item which is more
>>>>>>>>>>    frequently used only during the device errors.
>>>>>>>>>>
>>>>>>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
>>>>>>>>>>    id that we could represent is 63, its a kind of limitation which should
>>>>>>>>>>    be addressed before integration, I wonder if there is any suggestion?
>>>>>>>>>>    Kindly note that, multiple ioctls with each time representing a set of
>>>>>>>>>>    device(s) is not a choice because we need to make sure the readmirror
>>>>>>>>>>    changes happens in a commit transaction.
>>>>>>>>>>
>>>>>>>>>> v1->RFC v2:
>>>>>>>>>>   . Property is stored as a dev-tree item instead of root inode extended
>>>>>>>>>>     attribute.
>>>>>>>>>>   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
>>>>>>>>>>   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
>>>>>>>>>>
>>>>>>>>>> RFC->v1:
>>>>>>>>>>   Drops pid as one of the readmirror policy choices and as usual remains
>>>>>>>>>>   as default. And when the devid is reset the readmirror policy falls back
>>>>>>>>>>   to pid.
>>>>>>>>>>   Drops the mount -o readmirror idea, it can be added at a later point of
>>>>>>>>>>   time.
>>>>>>>>>>   Property now accepts more than 1 devid as readmirror device. As shown
>>>>>>>>>>   in the example above.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> This is a lot of infrastructure
>>>>>>
>>>>>> Ok. Any idea on a better implementation?
>>>>>> How about extended attribute approach? v1 patches proposed
>>>>>> it, but it abused the extended attribute as commented here [1]
>>>>>> and v2 got changed to an item-key.
>>>>>>
>>>>>> [1]
>>>>>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
>>>>>>
>>>>>
>>>>> That's a NAK on the prop interface.  This is a fs wide policy, not a
>>>>> directory/inode policy.
>>>>>
>>>>>>
>>>>>>>>> to just change which mirror we read to based on
>>>>>>>>> some arbitrary user policy.  I assume this is to solve the case where you have
>>>>>>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
>>>>>>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
>>>>>>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
>>>>>>>>> particular obscure setup.  Thanks,
>>>>>>>>>
>>>>>>>>> Josef
>>>>>>>>
>>>>>>>> Not commenting on the code itself, but as a user I see this SSD RAID1
>>>>>>>> acceleration as a future much have feature. It's only obscure at the
>>>>>>>> moment because we don't have code to take advantage of it. But on
>>>>>>>> large btrfs filesystems with hundreds of GB of metadata, like I have
>>>>>>>> for backups, the usability of the filesystem is dramatically improved
>>>>>>>> having the metadata on an SSD( though currently only half of the time
>>>>>>>> due to the even/odd pid distribution.)
>>>>>>>
>>>>>>> But that's different from a mirror.  100% it would be nice to say "put my
>>>>>>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
>>>>>>> patch is specifically about changing which drive we choose in a mirrored setup,
>>>>>>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
>>>>>>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
>>>>>>> but we can accomplish that by just adding a check for nonrotational in the code,
>>>>>>> and then we don't have to encode all this nonsense in the file system.  Thanks,
>>>>>>
>>>>>> I wrote about the readmirror policy framework here[2],
>>>>>> I forgot to link it here, sorry about that, my mistake.
>>>>>>
>>>>>> [2]
>>>>>>
>>>>>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
>>>>>>
>>>>>> Readmirror policy is for raid1, raid10 and future N way mirror.
>>>>>> Yes for now its only for raid1.
>>>>>>
>>>>>> Here the idea is to create a framework so that readmirror policy
>>>>>> can be configured as needed. And nonrotational can be one such policy.
>>>>>>
>>>>>> The example of hard-coded nonrotational policy does not work in case
>>>>>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
>>>>>> device, as all these are still nonrotational devices. So hard-coded
>>>>>> policy is not a good idea. If we have to hardcode then there is Q-depth
>>>>>> based readmirror routing is better (patch in the ML), but that is
>>>>>> not good enough, because some configs wants it based on the disk-LBA
>>>>>> so that SAN storage target cache is balanced and not duplicated.
>>>>>> So in short it must be a configurable policy.
>>>>>>
>>>>>
>>>>> Again, if you are mixing disk types you likely always want non-rotational, but
>>>>> still mixing different speed devices in a mirror setup is just asking for weird
>>>>> latency problems.  I don't think solving this use case is necessary.  If you mix
>>>>> ssd + network device in a serious production setup then you probably should be
>>>>> fired cause you don't know what you are doing.  Having the generic
>>>>> "nonrotational gets priority" is going to cover 99% of the actual use cases that
>>>>> make sense.
>>>>>
>>>>> The SAN usecase I can sort of see, but again I don't feel like it's a problem we
>>>>> need to solve with on-disk format.  Add a priority to sysfs so you can change it
>>>>> with udev or something on the fly.  Thanks,
>>>>>
>>>>
>>>> Ok.
>>>> Sysfs is fine however we need configuration to be persistent across reboots.
>>>> Any idea?
>>>>
>>>
>>> Udev rules.  Thanks,
>>>
>>
>>   Josef, configs moving along with the luns in san seems to be more
>>   easy to manage, otherwise when the host fails, each potential new
>>   server has to be pre-configured with the udev rules.
>>
> 
> It's 2019, if people haven't figured out how to do persistent configuration by
> now then all hope is lost.  Facebook persistently configures millions of
> machines, I'm sure people can figure out how to make sure a udev rule ends up on
> the right host connected to a SAN that doesn't move.  Thanks,
> 
> Josef
> 

yeah. sysfs interface should be fine for now, and based on the
usage feedback we could consider persistent storage for the readmirror.

Typical to WORM (Write Once Read Many times) setups they mainly need
good read performance as data is imported once from somewhere else.
Just out of curiosity ran read performance checks using the readmirror 
property on ssd and nvme device.

$ btrfs fi show
Label: none  uuid: e24bd8b5-a9e9-41d4-b28e-e95c0c545466
	Total devices 2 FS bytes used 391.64MiB
	devid    1 size 447.13GiB used 2.01GiB path /dev/sdf
	devid    2 size 5.82TiB used 2.01GiB path /dev/nvme0n1

$ cat /sys/block/sdf/queue/rotational
0
$ cat /sys/block/nvme0n1/queue/rotational
0

$ dropcache; sleep 3; btrfs prop set /btrfs readmirror devid:2 && time 
md5sum /btrfs/fOH2
6129ccf74f7a761e0c3e096e051ba7a2  /btrfs/fOH2

real	0m0.725s
user	0m0.604s
sys	0m0.113s

$ dropcache; sleep 3; btrfs prop set /btrfs readmirror devid:1 && time 
md5sum /btrfs/fOH2
6129ccf74f7a761e0c3e096e051ba7a2  /btrfs/fOH2

real	0m1.125s
user	0m0.643s
sys	0m0.087s

nvme provides ~40% improvement for 400mb read io, however there are no
other IOs. The question will be what kind of read io load balancer will
be required at the full throttle of the preferred readmirror
device IO Q depth [1]. Which means a heuristic is required to juggle
around the policies and will be another readmirror policy to be part
of the same framework when required.

[1]
$ cat /sys/block/nvme0n1/queue/nr_requests
1023
$ cat /sys/block/sdf/queue/nr_requests
64

Thanks, Anand
Anand Jain Sept. 24, 2019, 2:27 p.m. UTC | #13
On 9/16/19 4:19 PM, Anand Jain wrote:
> On 12/9/19 6:13 PM, Josef Bacik wrote:
>> On Thu, Sep 12, 2019 at 06:10:08PM +0800, Anand Jain wrote:
>>>
>>>
>>>> On 12 Sep 2019, at 6:03 PM, Josef Bacik <josef@toxicpanda.com> wrote:
>>>>
>>>> On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
>>>>>
>>>>>
>>>>>> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
>>>>>>
>>>>>> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
>>>>>>>
>>>>>>>
>>>>>>> Thanks for the comments. More below.
>>>>>>>
>>>>>>> On 12/9/19 3:16 AM, Josef Bacik wrote:
>>>>>>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
>>>>>>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik 
>>>>>>>>> <josef@toxicpanda.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
>>>>>>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() 
>>>>>>>>>>> uses
>>>>>>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
>>>>>>>>>>>
>>>>>>>>>>> This patch introduces a framework so that we can add policies 
>>>>>>>>>>> to determine
>>>>>>>>>>> the %mirror_num. And also adds the devid as the readmirror 
>>>>>>>>>>> policy.
>>>>>>>>>>>
>>>>>>>>>>> The new property is stored as an item in the device tree as 
>>>>>>>>>>> show below.
>>>>>>>>>>>     (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, 
>>>>>>>>>>> devid)
>>>>>>>>>>>
>>>>>>>>>>> To be able to set and get this new property also introduces 
>>>>>>>>>>> new ioctls
>>>>>>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The 
>>>>>>>>>>> ioctl argument
>>>>>>>>>>> is defined as
>>>>>>>>>>>         struct btrfs_ioctl_readmirror_args {
>>>>>>>>>>>                 __u64 type; /* RW */
>>>>>>>>>>>                 __u64 device_bitmap; /* RW */
>>>>>>>>>>>         }
>>>>>>>>>>>
>>>>>>>>>>> An usage example as follows:
>>>>>>>>>>>         btrfs property set /btrfs readmirror devid:1,3
>>>>>>>>>>>         btrfs property get /btrfs readmirror
>>>>>>>>>>>           readmirror devid:1 3
>>>>>>>>>>>         btrfs property set /btrfs readmirror ""
>>>>>>>>>>>         btrfs property get /btrfs readmirror
>>>>>>>>>>>           readmirror default
>>>>>>>>>>>
>>>>>>>>>>> This patchset has been tested completely, however marked as 
>>>>>>>>>>> RFC for the
>>>>>>>>>>> following reasons and comments on them (or any other) are 
>>>>>>>>>>> appreciated as
>>>>>>>>>>> usual.
>>>>>>>>>>> . The new objectid is defined as
>>>>>>>>>>>       #define BTRFS_READMIRROR_OBJECTID -1ULL
>>>>>>>>>>>    Need consent we are fine to use this value, and with this 
>>>>>>>>>>> value it
>>>>>>>>>>>    shall be placed just before the DEV_STATS_OBJECTID item 
>>>>>>>>>>> which is more
>>>>>>>>>>>    frequently used only during the device errors.
>>>>>>>>>>>
>>>>>>>>>>> .  I am using a u64 bitmap to represent the devices id, so 
>>>>>>>>>>> the max device
>>>>>>>>>>>    id that we could represent is 63, its a kind of limitation 
>>>>>>>>>>> which should
>>>>>>>>>>>    be addressed before integration, I wonder if there is any 
>>>>>>>>>>> suggestion?
>>>>>>>>>>>    Kindly note that, multiple ioctls with each time 
>>>>>>>>>>> representing a set of
>>>>>>>>>>>    device(s) is not a choice because we need to make sure the 
>>>>>>>>>>> readmirror
>>>>>>>>>>>    changes happens in a commit transaction.
>>>>>>>>>>>
>>>>>>>>>>> v1->RFC v2:
>>>>>>>>>>>   . Property is stored as a dev-tree item instead of root 
>>>>>>>>>>> inode extended
>>>>>>>>>>>     attribute.
>>>>>>>>>>>   . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to 
>>>>>>>>>>> BTRFS_DEV_STATE_READ_PREFERRED.
>>>>>>>>>>>   . Changed format specifier from devid1,2,3.. to devid:1,2,3..
>>>>>>>>>>>
>>>>>>>>>>> RFC->v1:
>>>>>>>>>>>   Drops pid as one of the readmirror policy choices and as 
>>>>>>>>>>> usual remains
>>>>>>>>>>>   as default. And when the devid is reset the readmirror 
>>>>>>>>>>> policy falls back
>>>>>>>>>>>   to pid.
>>>>>>>>>>>   Drops the mount -o readmirror idea, it can be added at a 
>>>>>>>>>>> later point of
>>>>>>>>>>>   time.
>>>>>>>>>>>   Property now accepts more than 1 devid as readmirror 
>>>>>>>>>>> device. As shown
>>>>>>>>>>>   in the example above.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> This is a lot of infrastructure
>>>>>>>
>>>>>>> Ok. Any idea on a better implementation?
>>>>>>> How about extended attribute approach? v1 patches proposed
>>>>>>> it, but it abused the extended attribute as commented here [1]
>>>>>>> and v2 got changed to an item-key.
>>>>>>>
>>>>>>> [1]
>>>>>>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/ 
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> That's a NAK on the prop interface.  This is a fs wide policy, not a
>>>>>> directory/inode policy.
>>>>>>
>>>>>>>
>>>>>>>>>> to just change which mirror we read to based on
>>>>>>>>>> some arbitrary user policy.  I assume this is to solve the 
>>>>>>>>>> case where you have
>>>>>>>>>> slow and fast disks, so you can always read from the fast 
>>>>>>>>>> disk?  And then it's
>>>>>>>>>> only used in RAID1, so the very narrow usecase of having a 
>>>>>>>>>> RAID1 setup with a
>>>>>>>>>> SSD and a normal disk?  I'm not seeing a point to this much 
>>>>>>>>>> code for one
>>>>>>>>>> particular obscure setup.  Thanks,
>>>>>>>>>>
>>>>>>>>>> Josef
>>>>>>>>>
>>>>>>>>> Not commenting on the code itself, but as a user I see this SSD 
>>>>>>>>> RAID1
>>>>>>>>> acceleration as a future much have feature. It's only obscure 
>>>>>>>>> at the
>>>>>>>>> moment because we don't have code to take advantage of it. But on
>>>>>>>>> large btrfs filesystems with hundreds of GB of metadata, like I 
>>>>>>>>> have
>>>>>>>>> for backups, the usability of the filesystem is dramatically 
>>>>>>>>> improved
>>>>>>>>> having the metadata on an SSD( though currently only half of 
>>>>>>>>> the time
>>>>>>>>> due to the even/odd pid distribution.)
>>>>>>>>
>>>>>>>> But that's different from a mirror.  100% it would be nice to 
>>>>>>>> say "put my
>>>>>>>> metadata on the ssd, data elsewhere".  That's not what this 
>>>>>>>> patch is about, this
>>>>>>>> patch is specifically about changing which drive we choose in a 
>>>>>>>> mirrored setup,
>>>>>>>> which is super unlikely to mirror a SSD with a slow drive, cause 
>>>>>>>> it's just going
>>>>>>>> to be slow no matter what.  Sure we could make it so reads 
>>>>>>>> always go to the SSD,
>>>>>>>> but we can accomplish that by just adding a check for 
>>>>>>>> nonrotational in the code,
>>>>>>>> and then we don't have to encode all this nonsense in the file 
>>>>>>>> system.  Thanks,
>>>>>>>
>>>>>>> I wrote about the readmirror policy framework here[2],
>>>>>>> I forgot to link it here, sorry about that, my mistake.
>>>>>>>
>>>>>>> [2]
>>>>>>>
>>>>>>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/ 
>>>>>>>
>>>>>>>
>>>>>>> Readmirror policy is for raid1, raid10 and future N way mirror.
>>>>>>> Yes for now its only for raid1.
>>>>>>>
>>>>>>> Here the idea is to create a framework so that readmirror policy
>>>>>>> can be configured as needed. And nonrotational can be one such 
>>>>>>> policy.
>>>>>>>
>>>>>>> The example of hard-coded nonrotational policy does not work in case
>>>>>>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME 
>>>>>>> block
>>>>>>> device, as all these are still nonrotational devices. So hard-coded
>>>>>>> policy is not a good idea. If we have to hardcode then there is 
>>>>>>> Q-depth
>>>>>>> based readmirror routing is better (patch in the ML), but that is
>>>>>>> not good enough, because some configs wants it based on the disk-LBA
>>>>>>> so that SAN storage target cache is balanced and not duplicated.
>>>>>>> So in short it must be a configurable policy.
>>>>>>>
>>>>>>
>>>>>> Again, if you are mixing disk types you likely always want 
>>>>>> non-rotational, but
>>>>>> still mixing different speed devices in a mirror setup is just 
>>>>>> asking for weird
>>>>>> latency problems.  I don't think solving this use case is 
>>>>>> necessary.  If you mix
>>>>>> ssd + network device in a serious production setup then you 
>>>>>> probably should be
>>>>>> fired cause you don't know what you are doing.  Having the generic
>>>>>> "nonrotational gets priority" is going to cover 99% of the actual 
>>>>>> use cases that
>>>>>> make sense.
>>>>>>
>>>>>> The SAN usecase I can sort of see, but again I don't feel like 
>>>>>> it's a problem we
>>>>>> need to solve with on-disk format.  Add a priority to sysfs so you 
>>>>>> can change it
>>>>>> with udev or something on the fly.  Thanks,
>>>>>>
>>>>>
>>>>> Ok.
>>>>> Sysfs is fine however we need configuration to be persistent across 
>>>>> reboots.
>>>>> Any idea?
>>>>>
>>>>
>>>> Udev rules.  Thanks,
>>>>
>>>
>>>   Josef, configs moving along with the luns in san seems to be more
>>>   easy to manage, otherwise when the host fails, each potential new
>>>   server has to be pre-configured with the udev rules.
>>>
>>
>> It's 2019, if people haven't figured out how to do persistent 
>> configuration by
>> now then all hope is lost.  Facebook persistently configures millions of
>> machines, I'm sure people can figure out how to make sure a udev rule 
>> ends up on
>> the right host connected to a SAN that doesn't move.  Thanks,
>>
>> Josef
>>
> 
> yeah. sysfs interface should be fine for now, and based on the
> usage feedback we could consider persistent storage for the readmirror.
> 
> Typical to WORM (Write Once Read Many times) setups they mainly need
> good read performance as data is imported once from somewhere else.
> Just out of curiosity ran read performance checks using the readmirror 
> property on ssd and nvme device.
> 
> $ btrfs fi show
> Label: none  uuid: e24bd8b5-a9e9-41d4-b28e-e95c0c545466
>      Total devices 2 FS bytes used 391.64MiB
>      devid    1 size 447.13GiB used 2.01GiB path /dev/sdf
>      devid    2 size 5.82TiB used 2.01GiB path /dev/nvme0n1
> 
> $ cat /sys/block/sdf/queue/rotational
> 0
> $ cat /sys/block/nvme0n1/queue/rotational
> 0
> 
> $ dropcache; sleep 3; btrfs prop set /btrfs readmirror devid:2 && time 
> md5sum /btrfs/fOH2
> 6129ccf74f7a761e0c3e096e051ba7a2  /btrfs/fOH2
> 
> real    0m0.725s
> user    0m0.604s
> sys    0m0.113s
> 
> $ dropcache; sleep 3; btrfs prop set /btrfs readmirror devid:1 && time 
> md5sum /btrfs/fOH2
> 6129ccf74f7a761e0c3e096e051ba7a2  /btrfs/fOH2
> 
> real    0m1.125s
> user    0m0.643s
> sys    0m0.087s
> 
> nvme provides ~40% improvement for 400mb read io, however there are no
> other IOs. The question will be what kind of read io load balancer will
> be required at the full throttle of the preferred readmirror
> device IO Q depth [1]. Which means a heuristic is required to juggle
> around the policies and will be another readmirror policy to be part
> of the same framework when required.
> 
> [1]
> $ cat /sys/block/nvme0n1/queue/nr_requests
> 1023
> $ cat /sys/block/sdf/queue/nr_requests
> 64
> 
> Thanks, Anand


Josef,

  Here I infer as follows .. I wonder if it makes sense.

  - Potential readmirror policies are: pid, devid/manual, lba/cache,
    Q-depth(patches are in the ml from facebook) each one of them has
    their own advantage so a framework is essential.

  - As the block device performance keeps evolving so its common to have
    heterogeneous block devices on the host. readmirror policy helps
    to configure for the best possible performance.

  - sysfs is a good interface for readmirror policy as in ioctl we can't
    pass all possible devices in the ioctl argument.

  - ondisk storage is essential for the readmirror, and key-item pair
    is the only way for its storage. (On the 2nd thought storage cannot
    be implemented at a later point as sysfs interface must be consistent
    in each version. So implementing together sysfs and ondisk key-item
    pair makes sense to me.)

Thanks, Anand