From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755811Ab0CQQ1v (ORCPT ); Wed, 17 Mar 2010 12:27:51 -0400 Received: from e23smtp09.au.ibm.com ([202.81.31.142]:48200 "EHLO e23smtp09.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755606Ab0CQQ1q (ORCPT ); Wed, 17 Mar 2010 12:27:46 -0400 Date: Wed, 17 Mar 2010 21:57:39 +0530 From: Balbir Singh To: Anthony Liguori Cc: Chris Webb , Avi Kivity , KVM development list , Rik van Riel , KAMEZAWA Hiroyuki , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter Message-ID: <20100317162739.GU18054@balbir.in.ibm.com> Reply-To: balbir@linux.vnet.ibm.com References: <20100315072214.GA18054@balbir.in.ibm.com> <4B9DE635.8030208@redhat.com> <20100315080726.GB18054@balbir.in.ibm.com> <4B9DEF81.6020802@redhat.com> <20100315202353.GJ3840@arachsys.com> <4B9EC60A.2070101@codemonkey.ws> <20100317151409.GY31148@arachsys.com> <4BA0FB83.1010502@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <4BA0FB83.1010502@codemonkey.ws> User-Agent: Mutt/1.5.20 (2009-08-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Anthony Liguori [2010-03-17 10:55:47]: > On 03/17/2010 10:14 AM, Chris Webb wrote: > >Anthony Liguori writes: > > > >>This really gets down to your definition of "safe" behaviour. As it > >>stands, if you suffer a power outage, it may lead to guest > >>corruption. > >> > >>While we are correct in advertising a write-cache, write-caches are > >>volatile and should a drive lose power, it could lead to data > >>corruption. Enterprise disks tend to have battery backed write > >>caches to prevent this. > >> > >>In the set up you're emulating, the host is acting as a giant write > >>cache. Should your host fail, you can get data corruption. > >Hi Anthony. I suspected my post might spark an interesting discussion! > > > >Before considering anything like this, we did quite a bit of testing with > >OSes in qemu-kvm guests running filesystem-intensive work, using an ipmitool > >power off to kill the host. I didn't manage to corrupt any ext3, ext4 or > >NTFS filesystems despite these efforts. > > > >Is your claim here that:- > > > > (a) qemu doesn't emulate a disk write cache correctly; or > > > > (b) operating systems are inherently unsafe running on top of a disk with > > a write-cache; or > > > > (c) installations that are already broken and lose data with a physical > > drive with a write-cache can lose much more in this case because the > > write cache is much bigger? > > This is the closest to the most accurate. > > It basically boils down to this: most enterprises use a disks with > battery backed write caches. Having the host act as a giant write > cache means that you can lose data. > Dirty limits can help control how much we lose, but also affect how much we write out. > I agree that a well behaved file system will not become corrupt, but > my contention is that for many types of applications, data lose == > corruption and not all file systems are well behaved. And it's > certainly valid to argue about whether common filesystems are > "broken" but from a purely pragmatic perspective, this is going to > be the case. > I think it is a trade-off for end users to decide on. cache=writeback does provide performance benefits, but can cause data loss. -- Three Cheers, Balbir