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

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Date:	Mon, 15 Mar 2010 20:43:07 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Chris Webb <chris@...chsys.com>, 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 Mon, Mar 15, 2010 at 06:43:06PM -0500, Anthony Liguori wrote:
> I knew someone would do this...
>
> 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.
>
> cache=writethrough provides a much stronger data guarantee.  Even in the  
> event of a host failure, data integrity will be preserved.

Actually cache=writeback is as safe as any normal host is with a
volatile disk cache, except that in this case the disk cache is
actually a lot larger.  With a properly implemented filesystem this
will never cause corruption.  You will lose recent updates after
the last sync/fsync/etc up to the size of the cache, but filesystem
metadata should never be corrupted, and data that has been forced to
disk using fsync/O_SYNC should never be lost either.  If it is that's
a bug somewhere in the stack, but in my powerfail testing we never did
so using xfs or ext3/4 after I fixed up the fsync code in the latter
two.

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