Return-Path: Received: from fieldses.org ([173.255.197.46]:41096 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751150AbbCTS0W (ORCPT ); Fri, 20 Mar 2015 14:26:22 -0400 Date: Fri, 20 Mar 2015 14:26:21 -0400 From: "J. Bruce Fields" To: Christoph Hellwig Cc: Marc Eshel , Anna Schumaker , linux-nfs@vger.kernel.org, linux-nfs-owner@vger.kernel.org, xfs@oss.sgi.com Subject: Re: [PATCH v3 3/3] NFSD: Add support for encoding multiple segments Message-ID: <20150320182621.GH2036@fieldses.org> References: <20150318185545.GF8818@fieldses.org> <5509E27C.3080004@Netapp.com> <20150318205554.GA10716@fieldses.org> <5509E824.6070006@Netapp.com> <20150318211144.GB10716@fieldses.org> <20150319153627.GA20852@fieldses.org> <20150320151718.GD2036@fieldses.org> <20150320162303.GA18786@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20150320162303.GA18786@infradead.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: 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.