Re: live saving of domU

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

Jayesh Salvi wrote:
>
>
> On 5/10/06, *Anthony Liguori* <aliguori@us.ibm.com 
> <mailto:aliguori@us.ibm.com>> wrote:
>
>     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.
>
>
> Thanks, that was informative. But the shutdown you are refering to 
> here is not the traditional shutdown of domU right? I mean 'xm 
> shutdown' will do proper shutdown of domU OS, the shutdown you are 
> referring while doing 'xm save' is different from that, I hope!

 From the perspective of the hypervisor, there isn't a significant 
difference between doing an "xm shutdown" or an "xm save".  Don't read 
too much into that though b/c the vast majority of both operations are 
performed in userspace.  Unfortunately, this lack of distinction in the 
hypervisor is what currently prevents light weight check pointing.

You could, theoritically, add another entry point in the guest kernel 
and use some xenstore magic to force the domain to spin in a loop after 
canonicalizing, pause it, switch a register value, then unpause it.  
Something like (in psuedo-code):

while (eax == 0) halt;

Right after canonicalizing.  Then pause, and do a 
getvcpuinfo/setvcpuinfo to change eax to 1.  Then unpause.  If you added 
another suspend entry point you could probably even avoid tearing down 
the devices...

Regards,

Anthony Liguori

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