Re: [PATCH v3 3/3] NFSD: Add support for encoding multiple segments

J. Bruce Fields
2015-03-20T18:26:24Z

On Fri, Mar 20, 2015 at 09:23:03AM -0700, Christoph Hellwig wrote:
> On Fri, Mar 20, 2015 at 11:17:18AM -0400, J. Bruce Fields wrote:
> > Maybe this is a question for xfs developers.
> >
> > So, we have a new READ_PLUS call that's basically just a version of READ
> > optimized for sparse files:
> >
> > http://tools.ietf.org/html/draft-ietf-nfsv4-minorversion2-33#section-15.10
> >
> > It allows an NFS server to return either file data (like a normal READ
> > call) or, at the server's discretion, records saying "this range of the
> > data is all zeroes".
> >
> > Anna tried implementing READ_PLUS for knfsd using
> > vfs_llseek(.,.,SEEK_HOLE) followed by an ordinary read if that
> > determines we're not at a hole.
> >
> > (Very) preliminary results suggest that's slower than a plain READ for
> > an xfs file with no holes. (And *much* slower in the ext4 case for some
> > reason.)
>
> It should be a fairly cheap operastion, and does extent tree operations
> that are pretty similar to an (uncached) read. Do you have profiles?
>
> > Is that expected, and should we be doing this some other way instead?
>
> Are the read cached or uncached?

I don't know, and don't have profiles. I'll either try to reproduce or
wait till Anna's back from vacation.

> If they are from pagecache just copying the zeroes is pretty much
> unbeatable compared to extent tree lookups, so we'd need a new page
> flag (difficult..) to see that a page is a hole (and then it would
> only work for the whole page), but for uncached reads an optimization
> would be to tell a read that it's an NFS READ_PLUS so that it could
> just read until it reach a hole, and then we'd need some way to
> communicate the hole size (or just fall back to SEEK_HOLE for that
> case).

Ugh, OK. We'll do some more tests before coming back to ask about
that....

--b.