[Qemu-devel] -snapshot

[Qemu-devel] -snapshot

[Amador Pahim]

Hello,

Is a bad idea if I run multiples "qemu" vms pointing to the same disk
img using "-snapshot" parameter? What kind of problem may I have?

Thanks,

Re: [Qemu-devel] -snapshot

[Stefan Hajnoczi]

On Thu, Dec 16, 2010 at 4:21 PM, Amador Pahim <amador@pahim.org> wrote:
> Is a bad idea if I run multiples "qemu" vms pointing to the same disk
> img using "-snapshot" parameter? What kind of problem may I have?

It should work fine.  -snapshot means that QEMU creates a temporary
qcow2 file backed by the disk image you've given it.  No changes will
be made to your disk image - it is read-only.

Stefan

Re: [Qemu-devel] -snapshot

[Amador Pahim]

Hello Stefan,

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?

Regards,
Pahim

On Thu, Dec 16, 2010 at 2:28 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Thu, Dec 16, 2010 at 4:21 PM, Amador Pahim <amador@pahim.org> wrote:
>> Is a bad idea if I run multiples "qemu" vms pointing to the same disk
>> img using "-snapshot" parameter? What kind of problem may I have?
>
> It should work fine.  -snapshot means that QEMU creates a temporary
> qcow2 file backed by the disk image you've given it.  No changes will
> be made to your disk image - it is read-only.
>
> Stefan
>

Re: [Qemu-devel] -snapshot

[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

Re: [Qemu-devel] -snapshot

[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.

Regards
Stefan

Re: [Qemu-devel] -snapshot

[Amador Pahim]

Thanks. Some stuff are clear now. I started a VDI Open Source project
(http://www.ucs.br/projeto/osdvt/) and your information will help a
lot.

Pahim

On Thu, Dec 16, 2010 at 4:16 PM, Stefan Weil <weil@mail.berlios.de> wrote:
> 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
>
>

Re: [Qemu-devel] -snapshot

[Kevin Wolf]

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