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Date: Mon, 13 Oct 2008 14:34:47 -0400
From: Rik van Riel <riel@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
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To: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, Ryan Harper <ryanh@us.ibm.com>, Laurent Vivier <Laurent.Vivier@bull.net>, kvm-devel <kvm-devel@lists.sourceforge.net>

Jamie Lokier wrote:
> Rik van Riel wrote:
>>> When cache=on, read requests may not actually go to the disk.  If a 
>>> previous read request (by some application on the system) has read the 
>>> same data, then it becomes a simple memcpy().  Also, the host IO 
>>> scheduler may do read ahead which means that the data may be available 
>> >from that. 
>>
>> This can be as much of a data integrity problem as
>> asynchronous writes, if various qemu/kvm guests are
>> accessing the same disk image with a cluster filesystem
>> like GFS.
> 
> If there are multiple qemu/kvm guests accessing the same disk image in
> a cluster, provided the host cluster filesystem uses a fully coherent
> protocol, ordinary cached reads should be fine.  (E.g. not NFS).

The problem is when the synchronization only happens in the guests,
which is a legitimate and common configuration.

Ie. the hosts just pass through the IO and the guests run a GFS
cluster.

Caching either reads or writes at the host level causes problems.

-- 
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