From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: JFYI: ext4 bug triggerable by kvm Date: Tue, 17 Aug 2010 18:01:34 +0300 Message-ID: <4C6AA44E.3020504@redhat.com> References: <4C694483.5010903@msgid.tls.msk.ru> <4C694E7D.3060600@codemonkey.ws> <20100816184237.GA16579@infradead.org> <4C69A0C4.2080102@codemonkey.ws> <20100817090755.GA11110@infradead.org> <4C6A86E4.9080600@codemonkey.ws> <20100817130702.GA16635@infradead.org> <4C6A9AB5.6050404@codemonkey.ws> <20100817142808.GA22412@infradead.org> <4C6A9F13.9030104@codemonkey.ws> <20100817144507.GA10280@infradead.org> <4C6AA28F.1000605@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , Michael Tokarev , KVM list , Kevin Wolf To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:65099 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751493Ab0HQPBl (ORCPT ); Tue, 17 Aug 2010 11:01:41 -0400 In-Reply-To: <4C6AA28F.1000605@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: On 08/17/2010 05:54 PM, Anthony Liguori wrote: > > This is simply unrealistic. O_SYNC might force data to be on a > platter when using a directly attached disk but many NAS's actually do > writeback caching and relying on having an UPS to preserve data > integrity. There's really no way in the general case to ensure that > data is actually on a platter once you've involved a complex storage > setup or you assume FUA That's fine. Memory backed up by a UPS is a disk platter as far as the user is concerned, if the NAS is reliable. > > Let me put it another way. If an admin knows the disks on a machine > have battery backed cache, he's likely to leave writeback caching > enabled. In this case, as far as the host is concerned, there is no cache. Data written is guaranteed to reach the disk eventually even without a flush. Hopefully the disk advertises itself as not having a volatile cache. -- error compiling committee.c: too many arguments to function