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. The change may be journalled, but it isn't guaranteed stable until fsync is run on the inode. NFS server operations commit the metadata changed by a modification through ->commit_metadata or sync_inode_metadata() before the response is sent back to the client, hence guaranteeing that i_version changes through the NFS server are stable and durable. This is not the case for normal operations done through the POSIX API - the journalling is asynchronous and the only durability guarantees are provided by fsync().... > That said, I suppose it is possible for us to bump the counter, hand > that new counter value out to a NFS client and then the box crashes > before it makes it to the journal. Yup, this has aways been a problem when you mix posix applications running on the NFS server modifying the same files as the NFS clients are accessing and requiring synchronisation. > Not sure how big a problem that really is. This coherency problem has always existed on the server side... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
