Re: [Qemu-devel] [RFC]VM live snapshot proposal

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Mar 03, 2014 at 01:13:41AM +0000, Huangpeng (Peter) wrote:

Just to summarize the idea of live savevm for people joining the
discussion:

It should be possible to save a snapshot of the guest (including memory,
devices, and disk) without noticable downtime.

The 'savevm' command pauses the guest until the snapshot has been
completed and therefore doesn't meet the requirements.

> Here I have another proposal, based on the live-migration scheme, add consistent 
> memory state tracking and saving.
> The idea is simple:
> 1.First round use live-migration to save all memory to a snapshot file.
> 2.intercept the action of memory-modify, save old pages to a temporary file and mark dirty-bits,
> 3.Merge temporary file to the original snapshot file
> 
> Detailed process:
> (1)Pause VM
> (2) Save the device status to a temporary file (live-migration already supported )
> (3) Make disk snapshot
> (4) Enable page dirty log and old dirty pages save function(which we need to add)
> (5) Resume VM
> (6) Begin the first round of iteration, we save the entire contents of the VM memory pages
> to the snapshot file
> (7) In the second round of iteration , we save the old page to the snapshot file
> (8) Merge data of device status which is pre-saved in temporary files to the snapshot file
> (8) End ram snapshot and some cleanup work
> 
> Due to memory-modifications may happen in kvm, qemu, or vhost, the key-part is how we
> can provide common page-modify-tracking-and-saving api, we completed a prototype by 
> simply add modified-page tracking/saving function in qemu, and it seems worked fine.

Yes, this is the tricky part.  To be honest, I think this is the reason
no one has submitted patches - it's a hard task and the win isn't that
great (you can already migrate to file).

But back to the options:

If the host has enough free memory to fork QEMU, a small helper process
can be used to save the copy-on-write memory snapshot (thanks to fork(2)
semantics).  The hard part about the fork(2) approach is that QEMU isn't
really designed to fork, so work is necessary to reach a quiescent state
for the child process.

If there is not enough memory to fork, then a synchronous approach to
catching guest memory writes is needed.  I'm not sure if a good
mechanism for that exists but the simplest would be mprotect(2) and a
signal handler (which will make the guest run very slowly).

Stefan