On Wed 29-03-17 13:54:31, Jeff Layton wrote: > On Wed, 2017-03-29 at 13:15 +0200, Jan Kara wrote: > > On Tue 21-03-17 14:46:53, Jeff Layton wrote: > > > On Tue, 2017-03-21 at 14:30 -0400, J. Bruce Fields wrote: > > > > On Tue, Mar 21, 2017 at 01:23:24PM -0400, Jeff Layton wrote: > > > > > On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote: > > > > > > - It's durable; the above comparison still works if there were reboots > > > > > > between the two i_version checks. > > > > > > - I don't know how realistic this is--we may need to figure out > > > > > > if there's a weaker guarantee that's still useful. Do > > > > > > filesystems actually make ctime/mtime/i_version changes > > > > > > atomically with the changes that caused them? What if a > > > > > > change attribute is exposed to an NFS client but doesn't make > > > > > > it to disk, and then that value is reused after reboot? > > > > > > > > > > > > > > > > Yeah, there could be atomicity there. If we bump i_version, we'll mark > > > > > the inode dirty and I think that will end up with the new i_version at > > > > > least being journalled before __mark_inode_dirty returns. > > > > > > > > So you think the filesystem can provide the atomicity? In more detail: > > > > > > > > > > Sorry, I hit send too quickly. That should have read: > > > > > > "Yeah, there could be atomicity issues there." > > > > > > I think providing that level of atomicity may be difficult, though > > > maybe there's some way to make the querying of i_version block until > > > the inode update has been journalled? > > > > Just to complement what Dave said from ext4 side - similarly as with XFS > > ext4 doesn't guarantee atomicity unless fsync() has completed on the file. > > Until that you can see arbitrary combination of data & i_version after the > > crash. We do take care to keep data and metadata in sync only when there > > are security implications to that (like exposing uninitialized disk blocks) > > and if not, we are as lazy as we can to improve performance... > > > > > > Yeah, I think what we'll have to do here is ensure that those > filesystems do an fsync prior to reporting the i_version getattr > codepath. It's not pretty, but I don't see a real alternative. Hum, so are we fine if i_version just changes (increases) for all inodes after a server crash? If I understand its use right, it would mean invalidation of all client's caches but that is not such a big deal given how frequent server crashes should be, right? Because if above is acceptable we could make reported i_version to be a sum of "superblock crash counter" and "inode i_version". We increment "superblock crash counter" whenever we detect unclean filesystem shutdown. That way after a crash we are guaranteed each inode will report new i_version (the sum would probably have to look like "superblock crash counter" * 65536 + "inode i_version" so that we avoid reusing possible i_version numbers we gave away but did not write to disk but still...). Thoughts? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
