Re: Revisiting parallel save/restore

On Wed, Apr 17, 2024 at 05:12:27PM -0600, Jim Fehlig via Devel wrote: > 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. Yes, we'll need to be supporting 'mapped-ram', so a good first step. A question is whether we make that feature mandatory for all save images, or implied by another feature (parallel save), or an directly controllable feature with opt-in. The former breaks back compat with existnig libvirt, while the latter 2 options are net new so don't have compat implications. In terms of actual data blocks written on disk mapped-ram should be be the same size, or smaller, than the existing format. In terms of logical file size, however, mapped-ram will almost always be larger. This is because mapped-ram will result in a file whose logical size matches the guest RAM size, plus some header overhead, while being sparse so not all blocks are written. If tools handling save images aren't sparse-aware this could come across as a surprise and even be considered a regression. Mapped ram is needed for parallel saves since it lets each thread write to a specific region of the file. Mapped ram is good for non-parallel saves too though, because the mapping of RAM into the file is aligned suitably to allow for O_DIRECT to be used. Currently libvirt has to tunnnel over its iohelper to futz alignment needed for O_DIRECT. This makes it desirable to use in general, but back compat hurts...