| Message ID | 20240705-mgtime-v3-5-85b2daa9b335@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | fs: multigrain timestamp redux | expand |
On 7/5/24 10:02 AM, Jeff Layton wrote: > Add a high-level document that describes how multigrain timestamps work, > rationale for them, and some info about implementation and tradeoffs. > > Signed-off-by: Jeff Layton <jlayton@kernel.org> > --- > Documentation/filesystems/multigrain-ts.rst | 120 ++++++++++++++++++++++++++++ > 1 file changed, 120 insertions(+) > > diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst > new file mode 100644 > index 000000000000..70d36955bb83 > --- /dev/null > +++ b/Documentation/filesystems/multigrain-ts.rst > @@ -0,0 +1,120 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +===================== > +Multigrain Timestamps > +===================== > + > +Introduction > +============ > +Historically, the kernel has always used a coarse time values to stamp used coarse time values > +inodes. This value is updated on every jiffy, so any change that happens > +within that jiffy will end up with the same timestamp. > + > +When the kernel goes to stamp an inode (due to a read or write), it first gets > +the current time and then compares it to the existing timestamp(s) to see > +whether anything will change. If nothing changed, then it can avoid updating > +the inode's metadata. > + > +Coarse timestamps are therefore good from a performance standpoint, since they > +reduce the need for metadata updates, but bad from the standpoint of > +determining whether anything has changed, since a lot of things can happen in a > +jiffy. > + > +They are particularly troublesome with NFSv3, where unchanging timestamps can > +make it difficult to tell whether to invalidate caches. NFSv4 provides a > +dedicated change attribute that should always show a visible change, but not > +all filesystems implement this properly, causing the NFS server to substitute > +the ctime in many cases. > + > +Multigrain timestamps aim to remedy this by selectively using fine-grained > +timestamps when a file has had its timestamps queried recently, and the current > +coarse-grained time does not cause a change. > + > +Inode Timestamps > +================ > +There are currently 3 timestamps in the inode that are updated to the current > +wallclock time on different activity: > + > +ctime: > + The inode change time. This is stamped with the current time whenever > + the inode's metadata is changed. Note that this value is not settable > + from userland. > + > +mtime: > + The inode modification time. This is stamped with the current time > + any time a file's contents change. > + > +atime: > + The inode access time. This is stamped whenever an inode's contents are > + read. Widely considered to be a terrible mistake. Usually avoided with > + options like noatime or relatime. > + > +Updating the mtime always implies a change to the ctime, but updating the > +atime due to a read request does not. > + > +Multigrain timestamps are only tracked for the ctime and the mtime. atimes are > +not affected and always use the coarse-grained value (subject to the floor). > + > +Inode Timestamp Ordering > +======================== > + > +In addition just providing info about changes to individual files, file > +timestamps also serve an important purpose in applications like "make". These > +programs measure timestamps in order to determine whether source files might be > +newer than cached objects. > + > +Userland applications like make can only determine ordering based on > +operational boundaries. For a syscall those are the syscall entry and exit > +points. For io_uring or nfsd operations, that's the request submission and > +response. In the case of concurrent operations, userland can make no > +determination about the order in which things will occur. > + > +For instance, if a single thread modifies one file, and then another file in > +sequence, the second file must show an equal or later mtime than the first. The > +same is true if two threads are issuing similar operations that do not overlap > +in time. > + > +If however, two threads have racing syscalls that overlap in time, then there > +is no such guarantee, and the second file may appear to have been modified > +before, after or at the same time as the first, regardless of which one was > +submitted first. > + > +Multigrain Timestamps > +===================== > +Multigrain timestamps are aimed at ensuring that changes to a single file are > +always recognizeable, without violating the ordering guarantees when multiple recognizable according to what I can find on the web. > +different files are modified. This affects the mtime and the ctime, but the > +atime will always use coarse-grained timestamps. > + > +It uses an unused bit in the i_ctime_nsec field to indicate whether the mtime > +or ctime has been queried. If either or both have, then the kernel takes > +special care to ensure the next timestamp update will display a visible change. > +This ensures tight cache coherency for use-cases like NFS, without sacrificing > +the benefits of reduced metadata updates when files aren't being watched. > + > +The Ctime Floor Value > +===================== > +It's not sufficient to simply use fine or coarse-grained timestamps based on > +whether the mtime or ctime has been queried. A file could get a fine grained > +timestamp, and then a second file modified later could get a coarse-grained one > +that appears earlier than the first, which would break the kernel's timestamp > +ordering guarantees. > + > +To mitigate this problem, we maintain a global floor value that ensures that > +this can't happen. The two files in the above example may appear to have been > +modified at the same time in such a case, but they will never show the reverse > +order. To avoid problems with realtime clock jumps, the floor is managed as a > +monotonic ktime_t, and the values are converted to realtime clock values as > +needed. > + > +Implementation Notes > +==================== > +Multigrain timestamps are intended for use by local filesystems that get > +ctime values from the local clock. This is in contrast to network filesystems > +and the like that just mirror timestamp values from a server. > + > +For most filesystems, it's sufficient to just set the FS_MGTIME flag in the > +fstype->fs_flags in order to opt-in, providing the ctime is only ever set via > +inode_set_ctime_current(). If the filesystem has a ->getattr routine that > +doesn't call generic_fillattr, then you should have it call fill_mg_cmtime to > +fill those values. >
diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst new file mode 100644 index 000000000000..70d36955bb83 --- /dev/null +++ b/Documentation/filesystems/multigrain-ts.rst @@ -0,0 +1,120 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Multigrain Timestamps +===================== + +Introduction +============ +Historically, the kernel has always used a coarse time values to stamp +inodes. This value is updated on every jiffy, so any change that happens +within that jiffy will end up with the same timestamp. + +When the kernel goes to stamp an inode (due to a read or write), it first gets +the current time and then compares it to the existing timestamp(s) to see +whether anything will change. If nothing changed, then it can avoid updating +the inode's metadata. + +Coarse timestamps are therefore good from a performance standpoint, since they +reduce the need for metadata updates, but bad from the standpoint of +determining whether anything has changed, since a lot of things can happen in a +jiffy. + +They are particularly troublesome with NFSv3, where unchanging timestamps can +make it difficult to tell whether to invalidate caches. NFSv4 provides a +dedicated change attribute that should always show a visible change, but not +all filesystems implement this properly, causing the NFS server to substitute +the ctime in many cases. + +Multigrain timestamps aim to remedy this by selectively using fine-grained +timestamps when a file has had its timestamps queried recently, and the current +coarse-grained time does not cause a change. + +Inode Timestamps +================ +There are currently 3 timestamps in the inode that are updated to the current +wallclock time on different activity: + +ctime: + The inode change time. This is stamped with the current time whenever + the inode's metadata is changed. Note that this value is not settable + from userland. + +mtime: + The inode modification time. This is stamped with the current time + any time a file's contents change. + +atime: + The inode access time. This is stamped whenever an inode's contents are + read. Widely considered to be a terrible mistake. Usually avoided with + options like noatime or relatime. + +Updating the mtime always implies a change to the ctime, but updating the +atime due to a read request does not. + +Multigrain timestamps are only tracked for the ctime and the mtime. atimes are +not affected and always use the coarse-grained value (subject to the floor). + +Inode Timestamp Ordering +======================== + +In addition just providing info about changes to individual files, file +timestamps also serve an important purpose in applications like "make". These +programs measure timestamps in order to determine whether source files might be +newer than cached objects. + +Userland applications like make can only determine ordering based on +operational boundaries. For a syscall those are the syscall entry and exit +points. For io_uring or nfsd operations, that's the request submission and +response. In the case of concurrent operations, userland can make no +determination about the order in which things will occur. + +For instance, if a single thread modifies one file, and then another file in +sequence, the second file must show an equal or later mtime than the first. The +same is true if two threads are issuing similar operations that do not overlap +in time. + +If however, two threads have racing syscalls that overlap in time, then there +is no such guarantee, and the second file may appear to have been modified +before, after or at the same time as the first, regardless of which one was +submitted first. + +Multigrain Timestamps +===================== +Multigrain timestamps are aimed at ensuring that changes to a single file are +always recognizeable, without violating the ordering guarantees when multiple +different files are modified. This affects the mtime and the ctime, but the +atime will always use coarse-grained timestamps. + +It uses an unused bit in the i_ctime_nsec field to indicate whether the mtime +or ctime has been queried. If either or both have, then the kernel takes +special care to ensure the next timestamp update will display a visible change. +This ensures tight cache coherency for use-cases like NFS, without sacrificing +the benefits of reduced metadata updates when files aren't being watched. + +The Ctime Floor Value +===================== +It's not sufficient to simply use fine or coarse-grained timestamps based on +whether the mtime or ctime has been queried. A file could get a fine grained +timestamp, and then a second file modified later could get a coarse-grained one +that appears earlier than the first, which would break the kernel's timestamp +ordering guarantees. + +To mitigate this problem, we maintain a global floor value that ensures that +this can't happen. The two files in the above example may appear to have been +modified at the same time in such a case, but they will never show the reverse +order. To avoid problems with realtime clock jumps, the floor is managed as a +monotonic ktime_t, and the values are converted to realtime clock values as +needed. + +Implementation Notes +==================== +Multigrain timestamps are intended for use by local filesystems that get +ctime values from the local clock. This is in contrast to network filesystems +and the like that just mirror timestamp values from a server. + +For most filesystems, it's sufficient to just set the FS_MGTIME flag in the +fstype->fs_flags in order to opt-in, providing the ctime is only ever set via +inode_set_ctime_current(). If the filesystem has a ->getattr routine that +doesn't call generic_fillattr, then you should have it call fill_mg_cmtime to +fill those values.
Add a high-level document that describes how multigrain timestamps work, rationale for them, and some info about implementation and tradeoffs. Signed-off-by: Jeff Layton <jlayton@kernel.org> --- Documentation/filesystems/multigrain-ts.rst | 120 ++++++++++++++++++++++++++++ 1 file changed, 120 insertions(+)