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Re: [Qemu-devel] [PATCH v3 0/9] block: generic image streaming

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v3 0/9] block: generic image streaming
Date: Wed, 14 Dec 2011 10:50:57 +0000

On Wed, Dec 14, 2011 at 1:42 AM, Yibin Shen <address@hidden> wrote:
> On Tue, Dec 13, 2011 at 11:18 PM, Stefan Hajnoczi <address@hidden> wrote:
>> On Tue, Dec 13, 2011 at 2:12 PM, Yibin Shen <address@hidden> wrote:
>>> all these patches looks good to me except one thing,
>>> when I run a "qemu-img commit" command,
>>> seems entire image(from start to end sector) will be write to the backing 
>>> file,
>>> I think what we really need is to commit only dirty sectors.
>>> also maybe we can use a writeback mechanism alternaively.
>> This series does not affect the behavior of "qemu-img commit".
>> Are you suggesting a new feature?  Please explain a little more.
> yes , I suggest a new feature,
> in some situation, we need to commit data in qed image to backing file.
> in old days ,people without copy-on-read, they only have copy-on-write,
> so 'qemu-img commit' commit all exsits data to backing file is rational.
> but with copy-on-read or image streaming,
> which means most data in the image may be identical with backing file,
> so commit entire image is a redundant operation.
> can we record the dirty bitmap for data differ from backing file,
> then in a commit operation, we only need to handle minimum data.

This operation is needed as a dual to the blockdev-snapshot-sync QMP
command.  It merges changes back into the base image while the VM is
running (it's basically an online version of qemu-img commit and only
copies changed data).

I agree that it's a useful operation and perhaps someone will work on
it in the future.


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