Re: i_version, NFSv4 change attribute

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Mon, 2009-11-23 at 11:44 -0500, J. Bruce Fields wrote: 
> If the side we want to optimize is the modifications, I wonder if we
> could do all the i_version increments on *read* of i_version?:
> 
> 	- writes (and other inode modifications) set an "i_version_dirty"
> 	  flag.
> 	- reads of i_version clear the i_version_dirty flag, increment
> 	  i_version, and return the result.
> 
> As long as the reader sees i_version_flag set only after it sees the
> write that caused it, I think it all works?

That probably won't make much of a difference to performance. Most NFSv4
clients will have every WRITE followed by a GETATTR operation in the
same compound, so your i_version_dirty flag will always immediately get
cleared.

The question is, though, why does the jbd2 machinery need to be engaged
on _every_ write? The NFS clients don't care if we lose an i_version
count due to a sudden server reboot, since that will trigger a rewrite
of the dirty data anyway once the server comes back up again.
As long as the i_version is guaranteed to be written to stable storage
on a successful call to fsync(), then the NFS data integrity
requirements are fully satisfied.

Trond