| Message ID | 20190829071731.11521-1-wqu@suse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | btrfs: volumes: Allow missing devices to be writeable | expand |
On Thu, Aug 29, 2019 at 03:17:31PM +0800, Qu Wenruo wrote: > [BUG] > There is a long existing bug that degraded mounted btrfs can allocate new > SINGLE/DUP chunks on a RAID1 fs: > #!/bin/bash > > dev1=/dev/test/scratch1 > dev2=/dev/test/scratch2 > mnt=/mnt/btrfs > > umount $mnt &> /dev/null > umount $dev1 &> /dev/null > umount $dev2 &> /dev/null > > dmesg -C > mkfs.btrfs -f -m raid1 -d raid1 $dev1 $dev2 > > wipefs -fa $dev2 > > mount -o degraded $dev1 $mnt > btrfs balance start --full $mnt > umount $mnt > echo "=== chunk after degraded mount ===" > btrfs ins dump-tree -t chunk $dev1 | grep stripe_len.*type > > The result fs will have chunks with SINGLE and DUP only: > === chunk after degraded mount === > length 33554432 owner 2 stripe_len 65536 type SYSTEM > length 1073741824 owner 2 stripe_len 65536 type DATA > length 1073741824 owner 2 stripe_len 65536 type DATA|DUP > length 219676672 owner 2 stripe_len 65536 type METADATA|DUP > length 33554432 owner 2 stripe_len 65536 type SYSTEM|DUP > > This behavior greatly breaks the RAID1 tolerance. > > Even with missing device replaced, if the device with DUP/SINGLE chunks > on them get missing, the whole fs can't be mounted RW any more. > And we already have reports that user even can't mount the fs as some > essential tree blocks got written to those DUP chunks. > > [CAUSE] > The cause is pretty simple, we treat missing devices as non-writable. > Thus when we need to allocate chunks, we can only fall back to single > device profiles (SINGLE and DUP). > > [FIX] > Just consider the missing devices as WRITABLE, so we allocate new chunks > on them to maintain old profiles. I'm not sure this is the best way to fix it, it makes the meaning of rw_devices ambiguous. A missing device is by definition not readable nor writeable. This should be tracked separatelly, ie. counting real devices that can be written and devices that can be considered for allocation (with a documented meaning that even missing devices are included).
On 2019/9/12 上午1:17, David Sterba wrote: > On Thu, Aug 29, 2019 at 03:17:31PM +0800, Qu Wenruo wrote: >> [BUG] >> There is a long existing bug that degraded mounted btrfs can allocate new >> SINGLE/DUP chunks on a RAID1 fs: >> #!/bin/bash >> >> dev1=/dev/test/scratch1 >> dev2=/dev/test/scratch2 >> mnt=/mnt/btrfs >> >> umount $mnt &> /dev/null >> umount $dev1 &> /dev/null >> umount $dev2 &> /dev/null >> >> dmesg -C >> mkfs.btrfs -f -m raid1 -d raid1 $dev1 $dev2 >> >> wipefs -fa $dev2 >> >> mount -o degraded $dev1 $mnt >> btrfs balance start --full $mnt >> umount $mnt >> echo "=== chunk after degraded mount ===" >> btrfs ins dump-tree -t chunk $dev1 | grep stripe_len.*type >> >> The result fs will have chunks with SINGLE and DUP only: >> === chunk after degraded mount === >> length 33554432 owner 2 stripe_len 65536 type SYSTEM >> length 1073741824 owner 2 stripe_len 65536 type DATA >> length 1073741824 owner 2 stripe_len 65536 type DATA|DUP >> length 219676672 owner 2 stripe_len 65536 type METADATA|DUP >> length 33554432 owner 2 stripe_len 65536 type SYSTEM|DUP >> >> This behavior greatly breaks the RAID1 tolerance. >> >> Even with missing device replaced, if the device with DUP/SINGLE chunks >> on them get missing, the whole fs can't be mounted RW any more. >> And we already have reports that user even can't mount the fs as some >> essential tree blocks got written to those DUP chunks. >> >> [CAUSE] >> The cause is pretty simple, we treat missing devices as non-writable. >> Thus when we need to allocate chunks, we can only fall back to single >> device profiles (SINGLE and DUP). >> >> [FIX] >> Just consider the missing devices as WRITABLE, so we allocate new chunks >> on them to maintain old profiles. > > I'm not sure this is the best way to fix it, it makes the meaning of > rw_devices ambiguous. A missing device is by definition not readable nor > writeable. > > This should be tracked separatelly, ie. counting real devices that can > be written and devices that can be considered for allocation (with a > documented meaning that even missing devices are included). > Indeed this sounds much better. I'd go that direction. Thanks, Qu
There is previous work [1] [1] https://lore.kernel.org/linux-btrfs/1461812780-538-1-git-send-email-anand.jain@oracle.com/ I guess it was on purpose that missing device is not part of alloc chunk, so to have lesser impact due to writehole bug. My target is to fix the writehole first, and then this and other bugs. > [FIX] > Just consider the missing devices as WRITABLE, so we allocate new chunks > on them to maintain old profiles. IMO. In a 3-disks raid1 when one of the disk fails, we still need the _new writes_ not to be degraded. Just use two available disks. This fix fails that idea which is being followed now. Thanks, Anand
On 2019/9/12 下午6:27, Anand Jain wrote: > > > There is previous work [1] > > [1] > https://lore.kernel.org/linux-btrfs/1461812780-538-1-git-send-email-anand.jain@oracle.com/ > > > > I guess it was on purpose that missing device is not part of > alloc chunk, so to have lesser impact due to writehole bug. > My target is to fix the writehole first, and then this and > other bugs. > >> [FIX] >> Just consider the missing devices as WRITABLE, so we allocate new chunks >> on them to maintain old profiles. > > IMO. In a 3-disks raid1 when one of the disk fails, we still > need the _new writes_ not to be degraded. Just use two available > disks. This fix fails that idea which is being followed now. So priority comes, use existing, and only when it's impossible, consider degraded/missing device? > > Thanks, Anand >
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 56f751192a6c..cc30b1fa9306 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -7002,6 +7002,18 @@ static int read_one_dev(struct extent_buffer *leaf, fill_device_from_item(leaf, dev_item, device); set_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state); + + /* + * We treat missing devices as writable, so that we can maintain + * the existing profiles without degrading to DUP/SINGLE. + */ + if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state)) { + set_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state); + list_add(&device->dev_alloc_list, + &fs_devices->alloc_list); + fs_devices->rw_devices++; + } + if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state) && !test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state)) { device->fs_devices->total_rw_bytes += device->total_bytes;
[BUG] There is a long existing bug that degraded mounted btrfs can allocate new SINGLE/DUP chunks on a RAID1 fs: #!/bin/bash dev1=/dev/test/scratch1 dev2=/dev/test/scratch2 mnt=/mnt/btrfs umount $mnt &> /dev/null umount $dev1 &> /dev/null umount $dev2 &> /dev/null dmesg -C mkfs.btrfs -f -m raid1 -d raid1 $dev1 $dev2 wipefs -fa $dev2 mount -o degraded $dev1 $mnt btrfs balance start --full $mnt umount $mnt echo "=== chunk after degraded mount ===" btrfs ins dump-tree -t chunk $dev1 | grep stripe_len.*type The result fs will have chunks with SINGLE and DUP only: === chunk after degraded mount === length 33554432 owner 2 stripe_len 65536 type SYSTEM length 1073741824 owner 2 stripe_len 65536 type DATA length 1073741824 owner 2 stripe_len 65536 type DATA|DUP length 219676672 owner 2 stripe_len 65536 type METADATA|DUP length 33554432 owner 2 stripe_len 65536 type SYSTEM|DUP This behavior greatly breaks the RAID1 tolerance. Even with missing device replaced, if the device with DUP/SINGLE chunks on them get missing, the whole fs can't be mounted RW any more. And we already have reports that user even can't mount the fs as some essential tree blocks got written to those DUP chunks. [CAUSE] The cause is pretty simple, we treat missing devices as non-writable. Thus when we need to allocate chunks, we can only fall back to single device profiles (SINGLE and DUP). [FIX] Just consider the missing devices as WRITABLE, so we allocate new chunks on them to maintain old profiles. Signed-off-by: Qu Wenruo <wqu@suse.com> --- fs/btrfs/volumes.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)