From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1KLNGX-00065y-Ih for qemu-devel@nongnu.org; Tue, 22 Jul 2008 15:11:17 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1KLNGW-00065N-8t for qemu-devel@nongnu.org; Tue, 22 Jul 2008 15:11:16 -0400 Received: from [199.232.76.173] (port=43699 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1KLNGV-00065G-Ux for qemu-devel@nongnu.org; Tue, 22 Jul 2008 15:11:16 -0400 Received: from il.qumranet.com ([212.179.150.194]:59181) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1KLNGV-00051z-48 for qemu-devel@nongnu.org; Tue, 22 Jul 2008 15:11:15 -0400 Message-ID: <488630CD.4070200@qumranet.com> Date: Tue, 22 Jul 2008 22:11:09 +0300 From: Avi Kivity 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> <4885E9C2.9080209@codemonkey.ws> In-Reply-To: <4885E9C2.9080209@codemonkey.ws> 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 Anthony Liguori wrote: > 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 ought to be on the driver level. SCSI and NCQ disks should report barrier support; old IDE should report no barriers unless the user sets dont_care_about_performance_and_have_unlimited_warranty=1. ext* should use barriers if available. Of course this is linux-kernel material, not really on topic for this list. >>> 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. Ah, okay then. Qemu should be written assuming the underlying layers are sane; trying to work around Linux bugs is madness. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.