Re: [Qemu-devel] [PATCH RFC 0/3] Checkpoint-assisted migration proposal

[Thomas Knauth]

Hi Amit,

On Tue, Sep 15, 2015 at 12:39 PM, Amit Shah <address@hidden> wrote:
> Could you please include a file in the docs/ directory that documents
> how this works, so it's easier to comment on the general idea?

sure, we will add this.

> From 'checkpointing', I was afraid this was going to use some
> checkpoint-restore framework, but instead it's a new checkpointing
> method that you're adding to qemu.
>
> Can you describe when checkpoints are taken, and what is checkpointed?
> How is it stored on the disk?

Checkpoints are taken after a migration (at the source). If a
checkpoint exists at the destination, the VM's state is reconstructed
from the local checkpoint as well as updated pages from the source.
This checkpoint-assisted migration can be faster, if network is the
bottleneck, and saves network bandwidth.

We can, in principle, reuse the existing checkpoint format of QEMU.
The current implementation writes its own checkpoint because it was
less effort on our side. We write the VM's main memory into a single
file.

> I'm sure the patches have all the details, but it's easier to check
> the soundness of the idea if there's a high-level doc that explains
> this, and then we can discuss the finer points over patches.

We've recently published a paper about the general idea and expected
benefits for a number of workloads (
http://se.inf.tu-dresden.de/pubs/papers/knauth2015vecycle.pdf )

> Overall, I think this approach can benefit some workloads, and since
> it's not affecting a lot of common code, we could look at adding it.
>
> Also, apologies for not getting to this earlier.

Kind regards,
Thomas.