On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote: > On Tue, Mar 21, 2017 at 06:45:00AM -0700, Christoph Hellwig wrote: > > On Mon, Mar 20, 2017 at 05:43:27PM -0400, J. Bruce Fields wrote: > > > To me, the interesting question is whether this allows us to turn on > > > i_version updates by default on xfs and ext4. > > > > XFS v5 file systems have it on by default. > > Great, thanks. > > > Although we'll still need to agree on the exact semantics of i_version > > before it's going to be useful. > > Once it's figured out maybe we should write it up for a manpage that > could be used if statx starts exposing it to userspace. > > A first attempt: > > - It's a u64. > > - It works for regular files and directories. (What about symlinks or > other special types?) > > - It changes between two checks if and only if there were intervening > data or metadata changes. The change will always be an increase, but > the amount of the increase is meaningless. > - NFS doesn't actually require that it increases, but I think it > should. I assume 64 bits means we don't need a discussion of > wraparound. I thought NFS spec required that you be able to recognize old change attributes so that they can be discarded. I could be wrong here though. I'd have to go back and look through the spec to be sure. > - AFS wants an actual counter: if you get i_version X, then > write twice, then get i_version X+2, you're allowed to assume > your writes were the only modifications. Let's ignore this > for now. In the future if someone explains how to count > operations, then we can extend the interface to tell the > caller it can get those extra semantics. > > - 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. 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. Not sure how big a problem that really is. > Am I missing any issues? > No, I think you have it covered, and that's pretty much exactly what I had in mind as far as semantics go. Thanks for writing it up! -- Jeff Layton <jlayton@xxxxxxxxxx>
