Re: [RFC] Consistent Snapshots Idea

[Avi Kivity <avi@redhat.com>]

On 11/21/2011 02:01 PM, Richard Laager wrote:
> I'm not an expert on the architecture of KVM, so perhaps this is a QEMU
> question. If so, please let me know and I'll ask on a different list.

It is a qemu question, yes (though fork()ing a guest also relates to kvm).

> Background:
>
> Assuming the block layer can make instantaneous snapshots of a guest's
> disk (e.g. lvcreate -s), one can get "crash consistent" (i.e. as if the
> guest crashed) snapshots. To get a "fully consistent" snapshot, you need
> to shutdown the guest. For production VMs, this is obviously not ideal.
>
> Idea:
>
> What if KVM/QEMU was to fork() the guest and shutdown one copy?
>
> KVM/QEMU would momentarily halt the execution of the guest and take a
> writable, instantaneous snapshot of each block device. Then it would
> fork(). The parent would resume execution as normal. The child would
> redirect disk writes to the snapshot(s). The RAM should have
> copy-on-write behavior as with any other fork()ed process. Other
> resources like the network, display, sound, serial, etc. would simply be
> disconnected/bit-bucketed. Finally, the child would resume guest
> execution and send the guest an ACPI power button press event. This
> would cause the guest OS to perform an orderly shutdown.
>
> I believe this would provide consistent snapshots in the vast majority
> of real-world scenarios in a guest OS and application-independent way.

Interesting idea.  Will the guest actually shut down nicely without a
network?  Things like NFS mounts will break.

> Implementation Nits:
>
>       * A timeout on the child process would likely be a good idea.
>       * It'd probably be best to disconnect the network (i.e. tell the
>         guest the cable is unplugged) to avoid long timeouts. Likewise
>         for the hardware flow-control lines on the serial port.

This is actually critical, otherwise the guest will shutdown(2) all
sockets and confuse the clients.

>       * For correctness, fdatasync()ing or similar might be necessary
>         after halting execution and before creating the snapshots.

Microsoft guests have an API to quiesce storage prior to a snapshot, and
I think there is work to bring this to Linux guests.  So it should be
possible to get consistent snapshots even without this, but it takes
more integration.

-- 
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