On Mon, Jan 15, 2024 at 02:22:47PM +0800, Peter Xu wrote: > 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?
The 'fd' protocol should support 'fixed-ram' too if passed a seekable FD. The 'file' protocol should be able to create save images compatible with older QEMU too IMHO. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
