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. -- 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
