Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764030AbZDAAOd (ORCPT ); Tue, 31 Mar 2009 20:14:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756929AbZDAAOW (ORCPT ); Tue, 31 Mar 2009 20:14:22 -0400 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:54528 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756770AbZDAAOV (ORCPT ); Tue, 31 Mar 2009 20:14:21 -0400 Subject: Re: range-based cache flushing (was Re: Linux 2.6.29) From: James Bottomley To: Jeff Garzik Cc: Ric Wheeler , Jens Axboe , Linus Torvalds , Theodore Tso , Ingo Molnar , Alan Cox , Arjan van de Ven , Andrew Morton , Peter Zijlstra , Nick Piggin , David Rees , Jesper Krogh , Linux Kernel Mailing List In-Reply-To: <49D11806.80408@garzik.org> References: <20090324093245.GA22483@elte.hu> <20090324101011.6555a0b9@lxorguk.ukuu.org.uk> <20090324103111.GA26691@elte.hu> <20090324132032.GK5814@mit.edu> <20090324184549.GE32307@mit.edu> <49C93AB0.6070300@garzik.org> <20090325093913.GJ27476@kernel.dk> <49CA86BD.6060205@garzik.org> <20090325194341.GB27476@kernel.dk> <49CA8ADA.3040709@redhat.com> <49CA9114.3040205@garzik.org> <49CA9324.6010407@redhat.com> <1238016170.3281.36.camel@localhost.localdomain> <49D11806.80408@garzik.org> Content-Type: text/plain Date: Wed, 01 Apr 2009 00:14:17 +0000 Message-Id: <1238544857.5000.9.camel@mulgrave.int.hansenpartnership.com> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 (2.22.3.1-1.fc9) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2821 Lines: 65 On Mon, 2009-03-30 at 15:05 -0400, Jeff Garzik wrote: > James Bottomley wrote: > > On Wed, 2009-03-25 at 16:25 -0400, Ric Wheeler wrote: > >> Jeff Garzik wrote: > >>> Ric Wheeler wrote:> And, as I am sure that you do know, to add insult > >>> to injury, FLUSH_CACHE > >>>> is per device (not file system). > > >>>> When you issue an fsync() on a disk with multiple partitions, you > >>>> will flush the data for all of its partitions from the write cache.... > >>> SCSI'S SYNCHRONIZE CACHE command already accepts an (LBA, length) > >>> pair. We could make use of that. > > >>> And I bet we could convince T13 to add FLUSH CACHE RANGE, if we could > >>> demonstrate clear benefit. > > >> How well supported is this in SCSI? Can we try it out with a commodity > >> SAS drive? > > > What do you mean by well supported? The way the SCSI standard is > > written, a device can do a complete cache flush when a range flush is > > requested and still be fully standards compliant. There's no easy way > > to tell if it does a complete cache flush every time other than by > > taking the firmware apart (or asking the manufacturer). > > Quite true, though wondering aloud... > > How difficult would it be to pass the "lower-bound" LBA to SYNCHRONIZE > CACHE, where "lower bound" is defined as the lowest sector in the range > of sectors to be flushed? Actually, the implementation is designed to allow this. The standard says if the number of blocks is zero that means flush from the specified LBA to the end of the device. The sync cache we currently use has LBA 0 and number of blocks zero (which means flush everything). > That seems like a reasonable optimization -- it gives the drive an easy > way to skip sync'ing sectors lower than the lower-bound LBA, if it is > capable. Otherwise, a standards-compliant firmware will behave as you > describe, and do what our code currently expects today -- a full cache > flush. > > This seems like a good way to speed up cache flush [on SCSI], while also > perhaps experimenting with a more fine-grained way to pass down write > barriers to the device. > > Not a high priority thing overall, but OTOH, consider the case of > placing your journal at the end of the disk. You could then issue a > cache flush with a non-zero starting offset: > > SYNCHRONIZE CACHE (max sectors - JOURNAL_SIZE, ~0) > > That should be trivial even for dumb disk firmwares to optimize. We could try it ... I'm still not sure how we'd tell the device is actually implementing it and not flushing the entire device. James -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/