Re: [RFC PATCH v1 00/26] migration: File based migration with multifd and fixed-ram

[Peter Xu]

On Thu, Apr 20, 2023 at 10:02:43AM +0100, Daniel P. Berrangé wrote:
> On Wed, Apr 19, 2023 at 03:07:19PM -0400, Peter Xu wrote:
> > On Wed, Apr 19, 2023 at 06:12:05PM +0100, Daniel P. Berrangé wrote:
> > > On Tue, Apr 18, 2023 at 03:26:45PM -0400, Peter Xu wrote:
> > > > On Tue, Apr 18, 2023 at 05:58:44PM +0100, Daniel P. Berrangé wrote:
> > > > > Libvirt has multiple APIs where it currently uses its migrate-to-file
> > > > > approach
> > > > > 
> > > > >   * virDomainManagedSave()
> > > > > 
> > > > >     This saves VM state to an libvirt managed file, stops the VM, and 
> > > > > the
> > > > >     file state is auto-restored on next request to start the VM, and 
> > > > > the
> > > > >     file deleted. The VM CPUs are stopped during both save + restore
> > > > >     phase
> > > > > 
> > > > >   * virDomainSave/virDomainRestore
> > > > > 
> > > > >     The former saves VM state to a file specified by the mgmt 
> > > > > app/user.
> > > > >     A later call to virDomaniRestore starts the VM using that saved
> > > > >     state. The mgmt app / user can delete the file state, or re-use
> > > > >     it many times as they desire. The VM CPUs are stopped during both
> > > > >     save + restore phase
> > > > > 
> > > > >   * virDomainSnapshotXXX
> > > > > 
> > > > >     This family of APIs takes snapshots of the VM disks, optionally
> > > > >     also including the full VM state to a separate file. The snapshots
> > > > >     can later be restored. The VM CPUs remain running during the
> > > > >     save phase, but are stopped during restore phase
> > > > 
> > > > For this one IMHO it'll be good if Libvirt can consider leveraging the 
> > > > new
> > > > background-snapshot capability (QEMU 6.0+, so not very new..).  Or is 
> > > > there
> > > > perhaps any reason why a generic migrate:fd approach is better?
> > > 
> > > I'm not sure I fully understand the implications of 'background-snapshot' 
> > > ?
> > > 
> > > Based on what the QAPI comment says, it sounds potentially interesting,
> > > as conceptually it would be nicer to have the memory / state snapshot
> > > represent the VM at the point where we started the snapshot operation,
> > > rather than where we finished the snapshot operation.
> > > 
> > > It would not solve the performance problems that the work in this thread
> > > was intended to address though.  With large VMs (100's of GB of RAM),
> > > saving all the RAM state to disk takes a very long time, regardless of
> > > whether the VM vCPUs are paused or running.
> > 
> > I think it solves the performance problem by only copy each of the guest
> > page once, even if the guest is running.
> 
> I think we're talking about different performance problems.
> 
> What you describe here is about ensuring the snapshot is of finite size
> and completes in linear time, by ensuring each page is written only
> once.
> 
> What I'm talking about is being able to parallelize the writing of all
> RAM, so if a single thread can saturate the storage, using multiple
> threads will make the overal process faster, even when we're only
> writing each page once.

It depends on how much we want it.  Here the live snapshot scenaior could
probably leverage a same multi-threading framework with a vm suspend case
because it can assume all the pages are static and only saved once.

But I agree it's at least not there yet.. so we can directly leverage
multifd at least for now.

> 
> > Different from mostly all the rest of "migrate" use cases, background
> > snapshot does not use the generic dirty tracking at all (for KVM that's
> > get-dirty-log), instead it uses userfaultfd wr-protects, so that when
> > taking the snapshot all the guest pages will be protected once.
> 
> Oh, so that means this 'background-snapshot' feature only works on
> Linux, and only when permissions allow it. The migration parameter
> probably should be marked with 'CONFIG_LINUX' in the QAPI schema
> to make it clear this is a non-portable feature.

Indeed, I can have a follow up patch for this.  But it'll be the same as
some other features, like, postcopy (and all its sub-features including
postcopy-blocktime and postcopy-preempt)?

> 
> > It guarantees the best efficiency of creating a snapshot with VM running,
> > afaict.  I sincerely think Libvirt should have someone investigating and
> > see whether virDomainSnapshotXXX() can be implemented by this cap rather
> > than the default migration.
> 
> Since the background-snapshot feature is not universally available,
> it will only ever be possible to use it as an optional enhancement
> with virDomainSnapshotXXX, we'll need the portable impl to be the
> default / fallback.

I am actually curious on how a live snapshot can be implemented correctly
if without something like background snapshot.  I raised this question in
another reply here:

https://lore.kernel.org/all/ZDWBSuGDU9IMohEf@x1n/

I was using fixed-ram and vm suspend as example, but I assume it applies to
any live snapshot that is based on current default migration scheme.

For a real live snapshot (not vm suspend), IIUC we have similar challenges.

The problem is when migration completes (snapshot taken) the VM is still
running with a live disk image.  Then how can we take a snapshot exactly at
the same time when we got the guest image mirrored in the vm dump?  What
guarantees that there's no IO changes after VM image created but before we
take a snapshot on the disk image?

In short, it's a question on how libvirt can make sure the VM image and
disk snapshot image be taken at exactly the same time for live.

Thanks,

-- 
Peter Xu