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Date: Thu, 16 Dec 2010 19:16:16 +0100
From: Stefan Weil <weil@mail.berlios.de>
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Subject: Re: [Qemu-devel] -snapshot
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To: Amador Pahim <amador@pahim.org>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org

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.

Regards
Stefan