Re: Small O_SYNC writes are no longer NFS_DATA_SYNC

[NeilBrown <neilb@suse.de>]

On Thu, 17 Mar 2011 22:25:08 -0400 Trond Myklebust
<Trond.Myklebust@netapp.com> wrote:

> On Fri, 2011-03-18 at 13:12 +1100, NeilBrown wrote:
> > On Thu, 17 Mar 2011 21:49:26 -0400 Trond Myklebust
> > <Trond.Myklebust@netapp.com> wrote:
> > 
> > > However we could adopt the Solaris convention of always starting
> > > writebacks with a FILE_SYNC, and then falling back to UNSTABLE for the
> > > second rpc call and all subsequent calls...
> > > 
> > 
> > That approach certainly has merit.
> > 
> > However, as we know from the wbc info whether the write is small and sync -
> > which is the only case where I think a STABLE write is needed - I cannot see
> > why you don't want to just use that information to guide the choice of
> > 'stable' or not ???
> 
> By far the most common case we would want to optimise for is the sync at
> close() or fsync() when you have written a small file (<= wsize). If we
> can't optimise for that case, then the optimisation isn't worth doing at
> all.

Fair point.  I hadn't thought of that.

> 
> The point is that in that particular case, the wbc doesn't help you at
> all since the limits are set at 0 and LLONG_MAX (see nfs_wb_all(),
> write_inode_now(),...)
> 

I would be trivial to use min(wbc->range_end, i_size_read(inode)) as the
upper bound when assessing the size of the range to compare with 'wsize'.

However that wouldn't address the case of a small append to a large file
which would also be good to optimise.

If you can detect the 'first' RPC reliably at the same time that you still
have access to the wbc information, then:

 if this is the first request in a writeback, and the difference beween
  the address of this page, and min(wbc->range_end, i_size_read(inode))
  is less than wsize, then make it a STABLE write

might be a heuristic that catches most interesting cases.
It might be a bit complex though.

I think we should in general err on the size of not using a STABLE write
when it might be useful rather than using a STABLE write when it is not
necessary as, while there a costs each way, I think the cost of incorrectly
using STABLE would be higher.

Thanks for the clarification.

NeilBrown