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Date: Fri, 17 Dec 2010 10:52:19 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] -snapshot
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To: Stefan Weil <weil@mail.berlios.de>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Amador Pahim <amador@pahim.org>

Am 16.12.2010 19:16, schrieb Stefan Weil:
> Am 16.12.2010 18:45, schrieb Stefan Hajnoczi:
>> On Thu, Dec 16, 2010 at 4:34 PM, Amador Pahim <amador@pahim.org> wrote:
>>> Thank you for your answer. Just one more question: If, while my
>>> "snapshot" vms are running, the main disk is modified by a non
>>> "snapshot" vm? For example, installing some extra software.. this can
>>> freeze vms or something?
>>
>> Correct, it is not safe to modify the base image while there is
>> another disk image backed off it.
>>
>> The reason for this is that the image only needs to store the changes
>> that were made on top of the base image. For anything which hasn't
>> been modified it will go back to the base image and read data from
>> there.
>>
>> If you modify the base image, then the filesystem in the base image is
>> not longer what your image file was created from and you have an
>> inconsistent view of the disk. It leads to odd behavior and is
>> unsafe.
>>
>> Stefan
> 
> There are useful scenarios where using the same disk
> simultaneously from a snapshot vm and a real system
> works.
> 
> If you have a hard disk with a dual boot configuration,
> it is sometimes useful to boot one configuration with
> the real system, then start qemu and boot the second
> configuration.
> 
> Even booting the same configuration twice
> (once with the real machine, once with qemu snapshot)
> is sometimes useful and works to a limited degree.
> It is a simple way to try new bootloader configurations
> or other boot setups.

Right, though this doesn't contradict what Stefan said. It only works
because in fact you don't modify the parts that your guest reads.

Kevin