From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1KLIXf-0007yR-Kb for qemu-devel@nongnu.org; Tue, 22 Jul 2008 10:08:39 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1KLIXd-0007tW-Eb for qemu-devel@nongnu.org; Tue, 22 Jul 2008 10:08:38 -0400 Received: from [199.232.76.173] (port=54210 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1KLIXd-0007t4-5m for qemu-devel@nongnu.org; Tue, 22 Jul 2008 10:08:37 -0400 Received: from py-out-1112.google.com ([64.233.166.178]:29208) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1KLIXc-0005mc-VW for qemu-devel@nongnu.org; Tue, 22 Jul 2008 10:08:37 -0400 Received: by py-out-1112.google.com with SMTP id p76so1352456pyb.10 for ; Tue, 22 Jul 2008 07:08:34 -0700 (PDT) Message-ID: <4885E9C2.9080209@codemonkey.ws> Date: Tue, 22 Jul 2008 09:08:02 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] qcow2 - safe on kill? safe on power fail? References: <47CF0E0C.9030807@quinthar.com> <47CF16C5.6040102@codemonkey.ws> <20080721181031.GA31773@shareable.org> <4884E6F1.5020205@codemonkey.ws> <20080721212604.GA2823@shareable.org> <48850A5A.3070106@codemonkey.ws> <488578CA.4000402@qumranet.com> In-Reply-To: <488578CA.4000402@qumranet.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Avi Kivity wrote: > Anthony Liguori wrote: >> >> Sure you can. If you don't have a battery backed disk cache and are >> using write-back (which is usually the default), you can definitely >> get corruption of the journal. Likewise, under the right scenarios, >> you will get journal corruption with the default mount options of >> ext3 because it doesn't use barriers. >> > > What about SCSI or SATA NCQ? On these, barriers don't impact > performance greatly. Good question, I don't know the answer. But ext3 doesn't autodetect SCSI/NCQ or anything. It disabled barriers by default. Some distros have changed this behavior historically (SLES I believe). >> This is very hard to see happen in practice though because these >> windows are very small--just like with QEMU. >> > > The exposure window with qemu is not small. It's as large as the page > cache of the host. Note I was careful to qualify my statements that cache=off was required. Regards, Anthony Liguori