On Thu, Jan 11, 2024 at 03:38:31PM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@redhat.com> writes: > > > On Mon, Nov 27, 2023 at 05:25:42PM -0300, Fabiano Rosas wrote: > >> Hi, > >> > >> In this v3: > >> > >> Added support for the "file:/dev/fdset/" syntax to receive multiple > >> file descriptors. This allows the management layer to open the > >> migration file beforehand and pass the file descriptors to QEMU. We > >> need more than one fd to be able to use O_DIRECT concurrently with > >> unaligned writes. > >> > >> Dropped the auto-pause capability. That discussion was kind of > >> stuck. We can revisit optimizations for non-live scenarios once the > >> series is more mature/merged. > >> > >> Changed the multifd incoming side to use a more generic data structure > >> instead of MultiFDPages_t. This allows multifd to restore the ram > >> using larger chunks. > >> > >> The rest are minor changes, I have noted them in the patches > >> themselves. > > > > Fabiano, > > > > Could you always keep a section around in the cover letter (and also in the > > upcoming doc file fixed-ram.rst) on the benefits of this feature? > > > > Please bare with me - I can start to ask silly questions. > > > > That's fine. Ask away! > > > I thought it was about "keeping the snapshot file small". But then when I > > was thinking the use case, iiuc fixed-ram migration should always suggest > > the user to stop the VM first before migration starts, then if the VM is > > stopped the ultimate image shouldn't be large either. > > > > Or is it about performance only? Where did I miss? > > Performance is the main benefit because fixed-ram enables the use of > multifd for file migration which would otherwise not be > parallelizable. To use multifd has been the direction for a while as you > know, so it makes sense. > > A fast file migration is desirable because it could be used for > snapshots with a stopped vm and also to replace the "exec:cat" hack > (this last one I found out about recently, Juan mentioned it in this > thread: https://lore.kernel.org/r/87cyx5ty26.fsf@secure.mitica).
I digged again the history, and started to remember the "live" migration case for fixed-ram. IIUC that is what Dan mentioned in below email regarding to the "virDomainSnapshotXXX" use case: https://lore.kernel.org/all/zd7mrgq+4qsdb...@redhat.com/ So IIUC "stopped VM" is not always the use case? If you agree with this, we need to document these two use cases clearly in the doc update: - "Migrate a VM to file, then destroy the VM" It should be suggested to stop the VM first before triggering such migration in this use case in the documents. - "Take a live snapshot of the VM" It'll be ideal if there is a portable interface to synchronously track dirtying of guest pages, but we don't... So fixed-ram seems to be the solution for such a portable solution for taking live snapshot across-platforms as long as async dirty tracking is still supported on that OS (aka KVM_GET_DIRTY_LOG). If async tracking is not supported, snapshot cannot be done live on the OS then, and one needs to use "snapshot-save". For this one, IMHO it would be good to mention (from QEMU perspective) the existance of background-snapshot even though libvirt didn't support it for some reason. Currently background-snapshot lacks multi-thread feature (nor O_DIRECT), though, so it may be less performant than fixed-ram. However if with all features there I believe that's even more performant. Please consider mention to a degree of detail on this. > > The size aspect is just an interesting property, not necessarily a > reason. See above on the 2nd "live" use case of fixed-ram. I think in that case, size is still a matter, then, because that one cannot stop the VM vcpus. > It's about having the file bounded to the RAM size. So a running > guest would not produce a continuously growing file. This is in contrast > with previous experiments (libvirt code) in using a proxy to put > multifd-produced data into a file. > > I'll add this^ information in a more organized matter to the docs and > cover letter. Let me know what else I need to clarify. Thanks. > > Some notes about fixed-ram by itself: > > This series also enables fixed-ram without multifd, which would only > take benefit of the size property. That is not part of our end goal > which is to have multifd + fixed-ram, but I kept it nonetheless because > it helps to debug/reason about the fixed-ram format without conflating > matters with multifd. Yes, makes sense. > > Fixed-ram without multifd also allows the file migration to take benefit > of direct io because the data portion of the file (pages) will be > written with alignment. This version of the series does not yet support > it, but I have a simple patch for the next version. > > I also had a - perhaps naive - idea that we could merge the io code + > fixed-ram first, to expedite things and later bring in the multifd and > directio enhancements, but the review process ended up not being that > modular. What's the review process issue you're talking about? If you can split the series that'll help merging for sure to me. IIRC there's complexity on passing the o-direct fds around, and not sure whether that chunk can be put at the last, similarly to split the multifd bits. One thing I just noticed is fixed-ram seems to be always preferred for "file:" migrations. Then can we already imply fixed-ram for "file" URIs? I'm even thinking whether we can make it the default and drop the fixed-ram capability: fixed-ram won't work besides file, and file won't make sense if not using offsets / fixed-ram. There's at least one problem, where we have released 8.2 with "file:", so it means it could break users already using "file:" there. I'm wondering whether that'll be worthwhile considering if we can drop the (seems redundant..) capability. What do you think? -- Peter Xu
