Re: Revisiting parallel save/restore

On Fri, Apr 26, 2024 at 10:03:29AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > > > 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... > > Note that QEMU doesn't support O_DIRECT without multifd. > > From mapped-ram patch series v4: > > - Dropped support for direct-io with fixed-ram _without_ multifd. This > is something I said I would do for this version, but I had to drop > it because performance is really bad. I think the single-threaded > precopy code cannot cope with the extra latency/synchronicity of > O_DIRECT. Note the reason for using O_DIRECT is *not* to make saving / restoring the guest VM faster. Rather it is to ensure that saving/restoring a VM does not trash the host I/O / buffer cache, which will negatively impact performance of all the *other* concurrently running VMs.