Re: [Xen-devel] live saving of domU
Jayesh Salvi wrote:
On 5/10/06, *Anthony Liguori* <aliguori@xxxxxxxxxx
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
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 closest thing you can achieve is a localhost migration. There are
some caveats to this, of course. The first is that you need to
much memory as the domain has available since you'll have a copy
domain created briefly while the migration takes
place. Migrations are
also quite intrusive since they involve tearing down and bringing
I've gotten a lot of requests for light weight checkpointing. AFAIK,
noone is actually working on it though.
> 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.
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx <mailto:Xen-devel@xxxxxxxxxxxxxxxxxxx>
Everything you can imagine is real
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