linux-kernel - Re: [PATCH][RF C/T/D] Unmapped page cache control

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Date:	Wed, 17 Mar 2010 10:55:47 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Chris Webb <chris@...chsys.com>
CC:	Avi Kivity <avi@...hat.com>, balbir@...ux.vnet.ibm.com,
	KVM development list <kvm@...r.kernel.org>,
	Rik van Riel <riel@...riel.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

On 03/17/2010 10:14 AM, Chris Webb wrote:
> Anthony Liguori<anthony@...emonkey.ws>  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
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