From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id 4CF9F6B01B3 for ; Wed, 17 Mar 2010 11:55:52 -0400 (EDT) Received: by pzk30 with SMTP id 30so639980pzk.12 for ; Wed, 17 Mar 2010 08:55:50 -0700 (PDT) Message-ID: <4BA0FB83.1010502@codemonkey.ws> Date: Wed, 17 Mar 2010 10:55:47 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter 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> In-Reply-To: <20100317151409.GY31148@arachsys.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Chris Webb Cc: Avi Kivity , balbir@linux.vnet.ibm.com, KVM development list , Rik van Riel , KAMEZAWA Hiroyuki , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" List-ID: 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. 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. Regards, Anthony Liguori -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org