From: Jan Kara Subject: Re: [RFC] ext4: Don't send extra barrier during fsync if there are no dirty pages. Date: Tue, 3 Aug 2010 15:21:54 +0200 Message-ID: <20100803132154.GG3322@quack.suse.cz> References: <20100429235102.GC15607@tux1.beaverton.ibm.com> <1272934667.2544.3.camel@mingming-laptop> <4BE02C45.6010608@redhat.com> <20100504154553.GA22777@infradead.org> <20100630124832.GA1333@thunk.org> <4C2B44C0.3090002@redhat.com> <20100630134429.GE1333@thunk.org> <20100721171609.GC1215@atrey.karlin.mff.cuni.cz> <20100803000939.GA2109@tux1.beaverton.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , tytso@mit.edu, Ric Wheeler , Christoph Hellwig , Mingming Cao , linux-ext4 , linux-kernel , Keith Mannthey , Mingming Cao To: "Darrick J. Wong" Return-path: Received: from cantor2.suse.de ([195.135.220.15]:49716 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750879Ab0HCNW1 (ORCPT ); Tue, 3 Aug 2010 09:22:27 -0400 Content-Disposition: inline In-Reply-To: <20100803000939.GA2109@tux1.beaverton.ibm.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon 02-08-10 17:09:39, Darrick J. Wong wrote: > On Wed, Jul 21, 2010 at 07:16:09PM +0200, Jan Kara wrote: > > Hi, > > > > > On Wed, Jun 30, 2010 at 09:21:04AM -0400, Ric Wheeler wrote: > > > > > > > > The problem with not issuing a cache flush when you have dirty meta > > > > data or data is that it does not have any tie to the state of the > > > > volatile write cache of the target storage device. > > > > > > We track whether or not there is any metadata updates associated with > > > the inode already; if it does, we force a journal commit, and this > > > implies a barrier operation. > > > > > > The case we're talking about here is one where either (a) there is no > > > journal, or (b) there have been no metadata updates (I'm simplifying a > > > little here; in fact we track whether there have been fdatasync()- vs > > > fsync()- worthy metadata updates), and so there hasn't been a journal > > > commit to do the cache flush. > > > > > > In this case, we want to track when is the last time an fsync() has > > > been issued, versus when was the last time data blocks for a > > > particular inode have been pushed out to disk. > > > > > > To use an example I used as motivation for why we might want an > > > fsync2(int fd[], int flags[], int num) syscall, consider the situation > > > of: > > > > > > fsync(control_fd); > > > fdatasync(data_fd); > > > > > > The first fsync() will have executed a cache flush operation. So when > > > we do the fdatasync() (assuming that no metadata needs to be flushed > > > out to disk), there is no need for the cache flush operation. > > > > > > If we had an enhanced fsync command, we would also be able to > > > eliminate a second journal commit in the case where data_fd also had > > > some metadata that needed to be flushed out to disk. > > Current implementation already avoids journal commit because of > > fdatasync(data_fd). We remeber a transaction ID when inode metadata has > > last been updated and do not force a transaction commit if it is already > > committed. Thus the first fsync might force a transaction commit but second > > fdatasync likely won't. > > We could actually improve the scheme to work for data as well. I wrote > > a proof-of-concept patches (attached) and they nicely avoid second barrier > > when doing: > > echo "aaa" >file1; echo "aaa" >file2; fsync file2; fsync file1 > > > > Ted, would you be interested in something like this? > > Well... on my fsync-happy workloads, this seems to cut the barrier count down > by about 20%, and speeds it up by about 20%. Nice, thanks for measurement. Honza -- Jan Kara SUSE Labs, CR