Re: [Xen-devel] live saving of domU

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ewan Mellor 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.
>   
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.
Regards,

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.
>
> Ewan.
>
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>   
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