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Date: Thu, 28 Oct 2010 11:15:01 +0100
Message-ID: <AANLkTin+JjVnmd8vBur0prcPL1v4=aNFMY=NSuV=cu7w@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: [PATCH v3 5/5] qed: Consistency check support
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org, Anthony Liguori <aliguori@us.ibm.com>, Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Avi Kivity <avi@redhat.com>

On Wed, Oct 27, 2010 at 4:52 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 22.10.2010 16:56, schrieb Stefan Hajnoczi:
>> This patch adds support for the qemu-img check command. =A0It also
>> introduces a dirty bit in the qed header to mark modified images as
>> needing a check. =A0This bit is cleared when the image file is closed
>> cleanly.
>>
>> If an image file is opened and it has the dirty bit set, a consistency
>> check will run and try to fix corrupted table offsets. =A0These
>> corruptions may occur if there is power loss while an allocating write
>> is performed. =A0Once the image is fixed it opens as normal again.
>>
>> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
>
> Hm, do I understand right that you fix the image and reset the dirty
> flag in the header during bdrv_open? So how does this work with
> migration, when the destination host opens the QED file before the
> source closes it? Doesn't the destination destroy the image by "fixing" i=
t?
>
> And even if that wasn't the case, clearing the flag means that the
> source might do new writes and thinks that the flag is still set. If the
> source crashes now, we may need a consistency check, but the dirty flag
> isn't set any more.
>
> Am I missing some detail?

You're right, migration is not supported.  This is also true for the
other image formats which cache metadata in memory though.

I am actually looking into migration next together with Adam Litke who
has already started.  We'll have to defer accessing the file until the
source has flushed/closed it.

Stefan