Re: [PATCH] xfs: fix i_version handling in xfs

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On Thu, 18 Aug 2022, Dave Chinner wrote:
> 
> > Maybe we should just go back to using ctime.  ctime is *exactly* what
> > NFSv4 wants, as long as its granularity is sufficient to catch every
> > single change.  Presumably XFS doesn't try to ensure this.  How hard
> > would it be to get any ctime update to add at least one nanosecond?
> > This would be enabled by a mount option, or possibly be a direct request
> > from nfsd.
> 
> We can't rely on ctime to be changed during a modification because
> O_NOCMTIME exists to enable "user invisible" modifications to be
> made. On XFS these still bump iversion, so while they are invisible
> to the user, they are still tracked by the filesystem and anything
> that wants to know if the inode data/metadata changed.
> 

O_NOCMTIME isn't mentioned in the man page, so it doesn't exist :-(

If they are "user invisible", should they then also be "NFS invisible"?
I think so.
As I understand it, the purpose of O_NOCMTIME is to allow optimisations
- do a lot of writes, then update the mtime, thus reducing latency.  I
think it is perfectly reasonable for all of that to be invisible to NFS.

NeilBrown



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