Re: [libvirt RFCv8 00/27] multifd save restore prototype

On Sat, May 07, 2022 at 03:42:53PM +0200, Claudio Fontana wrote: 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. 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 :|