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Date: Tue, 01 Jun 2010 14:46:25 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Default to cache=writeback
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To: Alexander Graf <agraf@suse.de>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, avi@redhat.com, qemu-devel@nongnu.org, aurelien@aurel32.net, hch@lst.de

Am 01.06.2010 13:31, schrieb Alexander Graf:
> 
> On 26.05.2010, at 21:28, Stefan Hajnoczi wrote:
> 
>> On Wed, May 26, 2010 at 7:13 PM, Alexander Graf <agraf@suse.de> wrote:
>>> In the previous discussion Anthony brought up the fact that cache=writeback is
>>> safe enough considering data integrity. If so, I don't see a reason not to use
>>> it as default, as it speeds up things a lot.
>>
>> cache=writeback is not a good default for qcow2:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=572825
>> http://wiki.qemu.org/Features/Qcow2DataIntegrity
>>
>> The actual qcow2 file itself can become corrupted because metadata
>> updates are not ordered or flushed with respect to each other or data.
>> This is more serious than losing data written after the last flush.
>>
>> I believe special case cache= defaults for raw vs qcow2 have been
>> mentioned before but I don't see any code in qemu.git currently that
>> ensures qcow2 is run safely by default.
> 
> Well since cache=writethrough is the default, it apparently isn't hit by the issues you mentioned.
> 
> Kevin, what's your opinion on this?

To be honest, I can't follow your argumentation. What you're saying is
basically "with the current default of writethrough it works, so it
can't be dangerous to change the default to writeback". Not sure how you
draw this conclusion.

Kevin