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 :-)