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Re: [Xen-devel] live saving of domU

To: Ewan Mellor <ewan@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] live saving of domU
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Wed, 10 May 2006 11:59:09 -0500
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jayesh Salvi <jayeshsalvi@xxxxxxxxx>
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Ewan Mellor wrote:
On Wed, May 10, 2006 at 10:40:31AM -0500, Jayesh Salvi wrote:


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


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

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

Moreover, you cannot dump the state of a domain after a pause and expect it to ever run again.

Guests are aware of the physical addresses of the memory that's been allocated to them. Because of this, to save a domain's state in a restorable way you need the guest to "canonicalize" itself. The only way to do this today is through a suspend operation which happens to be a subop of shutdown. Shutdowns are non-recoverable so you cannot use this as a snapshotting mechanism.

The closest thing you can achieve is a localhost migration. There are some caveats to this, of course. The first is that you need to have as much memory as the domain has available since you'll have a copy of the domain created briefly while the migration takes place. Migrations are also quite intrusive since they involve tearing down and bringing up all the devices.

I've gotten a lot of requests for light weight checkpointing. AFAIK, noone is actually working on it though.


Anthony Liguori

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.


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