Re: Revisiting parallel save/restore
On Thu, Apr 25, 2024 at 04:41:02PM -0600, Jim Fehlig via Devel wrote:
On 4/17/24 5:12 PM, Jim Fehlig wrote:
> Hi All,
>
> While Fabiano has been working on improving save/restore performance in
> qemu, I've been tinkering with the same in libvirt. The end goal is to
> introduce a new VIR_DOMAIN_SAVE_PARALLEL flag for save/restore, along
> with a VIR_DOMAIN_SAVE_PARAM_PARALLEL_CONNECTIONS parameter to specify
> the number of concurrent channels used for the save/restore. Recall
> Claudio previously posted a patch series implementing parallel
> save/restore completely in libvirt, using qemu's multifd functionality
> [1].
>
> A good starting point on this journey is supporting the new mapped-ram
> capability in qemu 9.0 [2]. Since mapped-ram is a new on-disk format, I
> assume we'll need a new QEMU_SAVE_VERSION 3 when using it? Otherwise I'm
> not sure how to detect if a saved image is in mapped-ram format vs the
> existing, sequential stream format.
While hacking on a POC, I discovered the save data cookie and assume the use
of mapped-ram could be recorded there?
The issue with that is the semantics around old libivrt loading
the new image. Old libvirt won't know to look for 'mapped-ram'
element/attribute in the XML cookie, so will think it is a
traditional image with hilariously predictable results :-)
Hence I think mapped-ram needs some addition/change to the
save image header that explicitly stops old libvirt trying
to consume it, thus the version bump you mention.
With regards,
Daniel
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