On Fri, Nov 14, 2014 at 09:22:14AM -0500, Trond Myklebust wrote: > On Thu, Nov 13, 2014 at 11:35 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Nov 13, 2014 at 07:43:33PM -0500, Trond Myklebust wrote: > >> On Thu, Nov 13, 2014 at 6:54 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> > On Thu, Nov 13, 2014 at 08:02:43AM -0500, Trond Myklebust wrote: > >> >> On Wed, Nov 12, 2014 at 7:28 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> >> > On Wed, Nov 12, 2014 at 09:26:16AM -0500, Trond Myklebust wrote: > >> >> >> On Wed, Nov 12, 2014 at 5:24 AM, Christoph Hellwig <hch@xxxxxx> wrote: > >> >> >> > On Wed, Nov 12, 2014 at 09:27:10AM +1100, Dave Chinner wrote: > >> >> >> >> To clarify what Christoph wrote, XFS updates i_version is updated > >> >> >> >> once per transaction that modifies the inode. So if a VFS level > >> >> >> >> operation results in multiple transactions then each transaction > >> >> >> >> will but the version. > >> >> >> >> > >> >> >> >> It was implemented that way because nobody could tell me what the > >> >> >> >> actual granularity requirement for change detection was. Hence what > >> >> >> >> I implemented was "be able to detect any persistent change that is > >> >> >> >> made" to cover all bases. > >> > > >> > FWIW, ext4 takes the same approach. See Ted's post today: > >> > > >> > http://www.spinics.net/lists/linux-ext4/msg46194.html > >> > > >> > "The inode_inc_iversion() in mark4_ext4_iloc_dirty() is probably not > >> > necessary, since we already should be incrementing i_version whenever > >> > ctime and mtime gets updated. The inode_inc_iversion() there is more > >> > of a "belt and suspenders" safety thing, on the theory that the extra > >> > bump in i_version won't hurt anything." > >> > > >> > >> It will hurt if it causes all the NFS clients to blow their caches > >> unnecessarily. > > > > Not my problem. We've just implemented what we were asked to > > implement. > > > >> Who asked for this? > > > > The only discussion where actual specifications were enumerated was > > during a thread about using i_version in the integrity measurement > > code (IMA subsystem). The NFSv4 requirements for the change counter > > were expressed here: > > > > https://lkml.org/lkml/2012/1/5/408 > > > > Don't blame us for implementing the vague "changes every time" > > requirements in a way that results in no chance of a persistent > > change to either data or metadata being missed by the filesystem. > > I'm not blaming anyone. I'm stating that I'm not aware of anybody who > needs to trace fiemap changes via the change attribute, and so I'm > asking where that requirement came from? It was seriously being considered - it appeared as a potential in NFSv4 draft specs for handling sparse file reads. Indeed, this draft directly mentions reading block maps from XFS and using it on the client side: http://tools.ietf.org/html/draft-hildebrand-nfsv4-read-sparse-00 "XFS supports the XFS_IOC_GETBMAP extended attribute, which returns the allocation information for a file. Clients can then use this information to only read allocated data blocks" Now, I know that was from 2010, and the eventual 2014 NFSv4.2 RFC doesn't have this in it, but go back 3-4 years ago when we were trying to work out to how make an on-disk version counter work sanely for all the different things we'd been hearing about were going to be necessary for NFSv4.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
