From mboxrd@z Thu Jan 1 00:00:00 1970 From: Samuel Thibault Subject: Re: IO caching on xen? Date: Wed, 16 Jul 2008 18:46:49 +0100 Message-ID: <20080716174649.GF14133@implementation.uk.xensource.com> References: <20080715113053.GH4529@implementation.uk.xensource.com> <20080715123253015.00000080236@djm-pc> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: <20080715123253015.00000080236@djm-pc> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer Cc: marian klein , "xen-devel@lists.xensource.com" , "xen-discuss@opensolaris.org" List-Id: xen-devel@lists.xenproject.org Dan Magenheimer, le Tue 15 Jul 2008 12:32:53 -0600, a =E9crit : > > marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a =E9crit : > > > I am worried that the xen (or any other VM) with zvol=20 > > device from ZFS > > > (or any other file system.) may remove the benefits of ZFS, > > > because the guest OS can write something to disk but the xen itsel= f > > > can do some buffering > > > or lazy writing and it may delay to write to ZFS backend. > >=20 > > Xen doesn't do any kind of buffering or lazy writing. It provides > > barriers to the guest, to ensure any needed coherency. >=20 > I'm not sure if this is related or not... >=20 > I was party to some internal irc where it was believed > to be the case that when a file-backed vbd using file:'...' > is used, dom0's page cache may contain buffer pages that > may be inconsistent with the disk Ah, indeed, in that case the page cache is used, and the barriers may not have proper effect. > whereas a file-backed vbd using tap:aio:'...' does not suffer from the > same problem. blktap uses O_DIRECT to avoid buffering completely indeed. Samuel