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Date: Thu, 24 Sep 2009 11:20:39 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Qemu savevm and CPU soft lockup
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Benjamin Cleyet-Marrel <bcm@accelance.fr>, qemu-devel@nongnu.org

Kevin Wolf wrote:
> Am 24.09.2009 00:28, schrieb Anthony Liguori:
>   
>> Jamie Lokier wrote:
>>     
>>> This is normal savevm behaviour, and it is exactly the reason why
>>> migrate-to-file is useful.  I would not be surprised if savevm is
>>> changed to use migrate-to-file internally at some point, but it does
>>> not look like happening soon.
>>>   
>>>       
>> It's the same infrastructure.  The reason savevm isn't live is that 
>> savevm stores it's data in a qcow2 file.  Right now the way qcow2 is 
>> structured, the snapshot has to be a fixed size and allocated at once.  
>> In order to make savevm live, we need a method to stream savevm data to 
>> a qcow2 file while still allowing other IO operations to that qcow2 file.
>>
>> I'm fairly sure this will require a change to the qcow2 format in order 
>> to support this.
>>     
>
> Hm, snapshots are nothing complicated from qcow2 perspective. Why do you
> think data needs to be fixed size?

What happens if you're in the middle of writing snapshot data and 
another cluster has to be allocated?  You'll need a way to store the 
snapshot data discontinuously.

Regards,

Anthony Liguori