Re: [RFC PATCH v3 00/30] migration: File based migration with multifd and fixed-ram

[Daniel P . Berrangé]

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 <peterx@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+4QsDBtKR@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
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