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Date: Tue, 1 Jun 2010 13:48:21 +0100
Message-ID: <AANLkTim9GxfKmqxb25beNIR999KA4IvP_ayZKWJTh8WN@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH] Default to cache=writeback
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Alexander Graf <agraf@suse.de>
Cc: kwolf@redhat.com, avi@redhat.com, qemu-devel@nongnu.org, aurelien@aurel32.net, hch@lst.de

On Tue, Jun 1, 2010 at 12:31 PM, Alexander Graf <agraf@suse.de> wrote:
>
> 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.

cache=writethrough is safe because it uses O_SYNC.

Stefan