On Mon, Jan 15, 2024 at 04:45:15PM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@redhat.com> writes: > > > 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. > > > > I'll include these in some form in the docs update.
Thanks. Fixed-ram should also need a separate file after the doc series applied. I'll try to prepare a pull this week so both fixed-ram and cpr should hopefully have place to hold its own file under docs/devel/migration/. PS: just in case it didn't land as soon, feel free to fetch migration-next of my github.com/peterx/qemu repo; I only put things there if they at least pass one round of CI, so the content should be relatively stable even though not fully guranteed. > > >> > >> 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? > > No issue per-se. I'm just mentioning that I split the series in a > certain way and no one seemed to notice. =) Oh :) > > Basically everything up until patch 10/30 is one chunk that is mostly > separate from multifd support (patches 11-22/30) and direct-io + fdset > (32-30/30). You can describe these in the cover letter. Personally I can always merge initial M patches when they're ready out of N; there'll be quite a few iochannel ones though in the first batch, so I'll check with Dan when the 1st chunk in ready stage on how it should go in. > > > > > 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. > > > > The logical sequence for merging in my view would be: > > 1 - file: support - Steven already did that > 2 - file: + fixed-ram > 2a- file: + fixed-ram + direct-io (optional, I will send a patch in v4) > 3 - file: + fixed-ram + multifd > 4 - file: + fixed-ram + multifd + direct-io (here we get the full perf. > benefits) > 5 - file:/dev/fdset + fixed-ram + multifd + direct-io (here we can go > enable libvirt support) Sounds good. Such planning is IMHO fine to be put into TODO section of devel/migration/fixed-ram.rst if you want, especially you already plan to post separate series. So you can start with the .rst file with the whole design all in; we can merge it first. You remove todos along with patchset goes in. Your call on how to do it. > > > 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? > > > > The file URI alone is good to replace the exec:cat trick. We'll need it > once we deprecate exec: to be able to do debugging of the stream. I didn't follow up much on Juan's previous plan to deprecate exec. But yeah anyway let's start with that cap. > > > 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
