Revisiting parallel save/restore

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. IIUC, mapped-ram cannot be used with the exiting 'fd:' migration URI and instead must use 'file:'. Does qemu advertise support for that? I couldn't find it. If not, 'file:' (available in qemu 8.2) predates mapped-ram, so in theory we could live without the advertisement. It's also not clear when we want to enable the mapped-ram capability. Should it always be enabled if supported by the underlying qemu? One motivation for creating the mapped-ram was to support direct-io of the migration stream in qemu, in which case it could be tied to VIR_DOMAIN_SAVE_BYPASS_CACHE. E.g. the mapped-ram capability is enabled when user specifies VIR_DOMAIN_SAVE_BYPASS_CACHE && user-provided path results in a seekable fd && qemu supports mapped-ram? Looking ahead, should the mapped-ram capability be required for supporting the VIR_DOMAIN_SAVE_PARALLEL flag? As I understand, parallel save/restore was another motivation for creating the mapped-ram feature. It allows multifd threads to write exclusively to the offsets provided by mapped-ram. Can multiple multifd threads concurrently write to an fd without mapped-ram? Regards, Jim [1] https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/3Y... [2] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/devel/migration/m...