Re: Revisiting parallel save/restore

Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > 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. Well, there's surely a performance degradation threshold that negates the benefits of perserving the caches. But maybe it's not as low as I initially thought then.