Re: [Xen-devel] live saving of domU

["Jayesh Salvi" <jayeshsalvi@xxxxxxxxx>]

On 5/10/06, Ewan Mellor <ewan@xxxxxxxxxxxxx> wrote:

On Wed, May 10, 2006 at 10:40:31AM -0500, Jayesh Salvi wrote:

> Hi,
>
> Could anyone tell me, why 'xm save' has live parameter set to false by
> default. From yesterday's patch ([PATCH] [XenD] Migration-related change)
> i guess this paramter is renamed to network.

This parameter has not been renamed -- the rename was for a similar flag
passed in to the device migration code, but the live flag for migration
remains.

> [Snip]
>
> I am interested in saving the state of a virtual machine to a file, but
> want to continue it running. I want to backup the state of the machine, so
> I want this to be unintrusive operation. I would like to pause the domU
> and save it to file but keeping it still in memory. After save to file is
> done I will unpause the domU.
>
> I don't see why this shouldn't be possible if live migration works so
> well.

The reason it's not supported at the moment is this: if you take a snapshot of
a VM, then run for a bit, and then try and run the snapshot against the same
filesystem that you were using before, you will inevitably corrupt the
filesystem.

If you had a way to snapshot the storage at the same time as the VM, then you
could make live snapshotting of VMs work properly.  As it is, this is not
integrated into Xend at the moment, and not supported because of the danger of
filesystem corruption if you don't know what you are doing.

OK. So as I understand it, we will need a snapshotting mechanism that does snapshot of storage and that of domU state in one single atomic operation. I will study the suspend, pause , migrate and shutdown semantics and see what would be the best to stop domU from modifying the disk "for a while" - during which we can have a consistent snapshot of storage. If nothing else works an explicit 'xm save' and behind the curtain 'xm restore' will do the job I guess - a bit intrusive solution it will be though.

If anyone has further ideas on this line, I am all ears.

Thanks!

Ewan.

-- 
Jayesh
------------------------------------------------------------------------
Everything you can imagine is real
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