[libvirt RFCv8 00/27] multifd save restore prototype

Thu May 12 17:04:19 UTC 2022

* Christophe Marie Francois Dupont de Dinechin (cdupontd at redhat.com) wrote:
> 
> 
> > On 10 May 2022, at 20:38, Daniel P. Berrangé <berrange at redhat.com> wrote:
> > 
> > On Sat, May 07, 2022 at 03:42:53PM +0200, Claudio Fontana wrote:
> >> This is v8 of the multifd save prototype, which fixes a few bugs,
> >> adds a few more code splits, and records the number of channels
> >> as well as the compression algorithm, so the restore command is
> >> more user-friendly.
> >> 
> >> It is now possible to just say:
> >> 
> >> virsh save mydomain /mnt/saves/mysave --parallel
> >> 
> >> virsh restore /mnt/saves/mysave --parallel
> >> 
> >> and things work with the default of 2 channels, no compression.
> >> 
> >> It is also possible to say of course:
> >> 
> >> virsh save mydomain /mnt/saves/mysave --parallel
> >>      --parallel-connections 16 --parallel-compression zstd
> >> 
> >> virsh restore /mnt/saves/mysave --parallel
> >> 
> >> and things also work fine, due to channels and compression
> >> being stored in the main save file.
> > 
> > For the sake of people following along, the above commands will
> > result in creation of multiple files
> > 
> >  /mnt/saves/mysave
> >  /mnt/saves/mysave.0
> >  /mnt/saves/mysave.1
> >  ....
> >  /mnt/saves/mysave.n
> > 
> > Where 'n' is the number of threads used.
> > 
> > Overall I'm not very happy with the approach of doing any of this
> > on the libvirt side.
> > 
> > Backing up, we know that QEMU can directly save to disk faster than
> > libvirt can. We mitigated alot of that overhead with previous patches
> > to increase the pipe buffer size, but some still remains due to the
> > extra copies inherant in handing this off to libvirt.
> > 
> > Using multifd on the libvirt side, IIUC, gets us better performance
> > than QEMU can manage if doing non-multifd write to file directly,
> > but we still have the extra copies in there due to the hand off
> > to libvirt. If QEMU were to be directly capable to writing to
> > disk with multifd, it should beat us again.
> > 
> > As a result of how we integrate with QEMU multifd, we're taking the
> > approach of saving the state across multiple files, because it is
> > easier than trying to get multiple threads writing to the same file.
> > It could be solved by using file range locking on the save file.
> > eg a thread can reserve say 500 MB of space, fill it up, and then
> > reserve another 500 MB, etc, etc. It is a bit tedious though and
> > won't align nicely. eg a 1 GB huge page, would be 1 GB + a few
> > bytes of QEMU RAM ave state header.
> 
> First, I do not understand why you would write things that are
> not page-aligned to start with? (As an aside, I don’t know
> how any dirty tracking would work if you do not keep things
> page-aligned).
> 
> Could uffd_register_memory accept a memory range that is
> not aligned? If so, when? Should that be specified in the
> interface?

This isn't about alignment in RAM, this is about alignment in the file.
We always write chunks of RAM that are aligned but where they end up in
the file is typically not aligned because the file is:
<header> <pages> <header> <pages>

especially in the non-multifd world.

> Second, instead of creating multiple files, why not write blocks
> at a location determined by an variable that you increment using
> atomic operations each time you need a new block? If you want to
> keep the blocks page-aligned in the file as well (which might help
> if you want to mmap the file at some point), then you need to
> build a map of the blocks that you tack at the end of the file.

If you're going to make it that complicated you may as well go back
to the old thing of writing into a qcow2.

Dave

> There may be good reasons not to do it that way, of course, but
> I am not familiar enough with the problem to know them.
> 
> > 
> > The other downside of multiple files is that it complicates life
> > for both libvirt and apps using libvirt. They need to be aware of
> > multiple files and move them around together. This is not a simple
> > as it might sound. For example, IIRC OpenStack would upload a save
> > image state into a glance bucket for later use. Well, now it needs
> > multiple distinct buckets and keep track of them all. It also means
> > we're forced to use the same concurrency level when restoring, which
> > is not neccessarily desirable if the host environment is different
> > when restoring. ie The original host might have had 8 CPUs, but the
> > new host might only have 4 available, or vica-verca.
> > 
> > 
> > I know it is appealing to do something on the libvirt side, because
> > it is quicker than getting an enhancement into new QEMU release. We
> > have been down this route before with the migration support in libvirt
> > in the past though, when we introduced the tunnelled live migration
> > in order to workaround QEMU's inability to do TLS encryption. I very
> > much regret that we ever did this, because tunnelled migration was
> > inherantly limited, so for example failed to work with multifd,
> > and failed to work with NBD based disk migration. In the end I did
> > what I should have done at the beginning and just added TLS support
> > to QEMU, making tunnelled migration obsolete, except we still have
> > to carry the code around in libvirt indefinitely due to apps using
> > it.
> > 
> > So I'm very concerned about not having history repeat itself and
> > give us a long term burden for  a solution that turns out to be a
> > evolutionary dead end.
> > 
> > I like the idea of parallel saving, but I really think we need to
> > implement this directly in QEMU, not libvirt. As previously
> > mentioned I think QEMU needs to get a 'file' migration protocol,
> > along with ability to directly map RAM  segments into fixed
> > positions in the file. The benefits are many
> > 
> > - It will save & restore faster because we're eliminating data
> >   copies that libvirt imposes via the iohelper
> > 
> > - It is simple for libvirt & mgmt apps as we still only
> >   have one file to manage
> > 
> > - It is space efficient because if a guest dirties a
> >   memory page, we just overwrite the existing contents
> >   at the fixed location in the file, instead of appending
> >   new contents to the file
> > 
> > - It will restore faster too because we only restore
> >   each memory page once, due to always overwriting the
> >   file in-place when the guest dirtied a page during save
> > 
> > - It can save and restore with differing number of threads,
> >   and can even dynamically change the number of threads
> >   in the middle of the save/restore operation 
> > 
> > As David G has pointed out the impl is not trivial on the QEMU
> > side, but from what I understand of the migration code, it is
> > certainly viable. Most importantly I think it puts us in a
> > better position for long term feature enhancements later by
> > taking the middle man (libvirt) out of the equation, letting
> > QEMU directly know what medium it is saving/restoring to/from.
> > 
> > 
> > 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 :|
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK