Re: [PATCH v0 0/4] background snapshot

[Dr. David Alan Gilbert]

* Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> Currently where is no way to make a vm snapshot without pausing a vm
> for the whole time until the snapshot is done. So, the problem is
> the vm downtime on snapshoting. The downtime value depends on the vmstate
> size, the major part of which is RAM and the disk performance which is
> used for the snapshot saving.
> 
> The series propose a way to reduce the vm snapshot downtime. This is done
> by saving RAM, the major part of vmstate, in the background when the vm
> is running.
> 
> The background snapshot uses linux UFFD write-protected mode for memory
> page access intercepting. UFFD write-protected mode was added to the linux 
> v5.7.
> If UFFD write-protected mode isn't available the background snapshot rejects 
> to
> run.

Hi Denis,
  I see Peter has responded to most of your patches, but just anted to
say thank you; but also to cc in a couple of other people;
David Hildenbrand (who is interested in unusual memory stuff) and
zhanghailiang who works on COLO which also does snapshotting and had
long wanted to use WP.

  2/4 was a bit big for my liking; please try and do it in smaller
chunks!

Dave

> How to use:
> 1. enable background snapshot capability
>    virsh qemu-monitor-command vm --hmp migrate_set_capability 
> background-snapshot on
> 
> 2. stop the vm
>    virsh qemu-monitor-command vm --hmp stop
> 
> 3. Start the external migration to a file
>    virsh qemu-monitor-command cent78-bs --hmp migrate exec:'cat > ./vm_state'
> 
> 4. Wait for the migration finish and check that the migration has completed 
> state.
> 
> Denis Plotnikov (4):
>   bitops: add some atomic versions of bitmap operations
>   migration: add background snapshot capability
>   migration: add background snapshot
>   background snapshot: add trace events for page fault processing
> 
>  qapi/migration.json     |   7 +-
>  include/exec/ramblock.h |   8 +
>  include/exec/ramlist.h  |   2 +
>  include/qemu/bitops.h   |  25 ++
>  migration/migration.h   |   1 +
>  migration/ram.h         |  19 +-
>  migration/savevm.h      |   3 +
>  migration/migration.c   | 142 +++++++++-
>  migration/ram.c         | 582 ++++++++++++++++++++++++++++++++++++++--
>  migration/savevm.c      |   1 -
>  migration/trace-events  |   2 +
>  11 files changed, 771 insertions(+), 21 deletions(-)
> 
> -- 
> 2.17.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK