On 4/26/24 4:07 AM, Daniel P. Berrangé wrote:
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 :-)
Haha :-). I need to recall we aim to support new-to-old migration upstream. We limit our downstream support scope, and this type of migration scenario is one that falls in the unsupported bucket. Regards, Jim