Re: live saving of domU

[Anthony Liguori <aliguori@us.ibm.com>]

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.
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>