Re: [PATCH v3 0/7] UFFD write-tracking migration/snapshots

[Dr. David Alan Gilbert]

* Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> Changes with v3:
> * coding style fixes to pass checkpatch test
> * qapi/migration.json: change 'track-writes-ram' capability
> *                      supported version to 6.0
> * fixes to commit message format
> 

cc'ing in COLO people, since they could use this as well. 
> This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's
> implemented in his series '[PATCH v0 0/4] migration: add background snapshot'.
> 
> Currently the only way to make (external) live VM snapshot is using existing
> dirty page logging migration mechanism. The main problem is that it tends to
> produce a lot of page duplicates while running VM goes on updating already
> saved pages. That leads to the fact that vmstate image size is commonly 
> several
> times bigger then non-zero part of virtual machine's RSS. Time required to
> converge RAM migration and the size of snapshot image severely depend on the
> guest memory write rate, sometimes resulting in unacceptably long snapshot
> creation time and huge image size.
> 
> This series propose a way to solve the aforementioned problems. This is done
> by using different RAM migration mechanism based on UFFD write protection
> management introduced in v5.7 kernel. The migration strategy is to 'freeze'
> guest RAM content using write-protection and iteratively release protection
> for memory ranges that have already been saved to the migration stream.
> At the same time we read in pending UFFD write fault events and save those
> pages out-of-order with higher priority.
> 
> How to use:
> 1. Enable write-tracking migration capability
>    virsh qemu-monitor-command <domain> --hmp migrate_set_capability.
> track-writes-ram on
> 
> 2. Start the external migration to a file
>    virsh qemu-monitor-command <domain> --hmp migrate exec:'cat > ./vm_state'
> 
> 3. Wait for the migration finish and check that the migration has completed.
> state.
> 
> Andrey Gruzdev (7):
>   introduce 'track-writes-ram' migration capability
>   introduce UFFD-WP low-level interface helpers
>   support UFFD write fault processing in ram_save_iterate()
>   implementation of write-tracking migration thread
>   implementation of vm_start() BH
>   the rest of write tracking migration code
>   introduce simple linear scan rate limiting mechanism
> 
>  include/exec/memory.h |   7 +
>  migration/migration.c | 338 +++++++++++++++++++++++++++++++-
>  migration/migration.h |   4 +
>  migration/ram.c       | 439 +++++++++++++++++++++++++++++++++++++++++-
>  migration/ram.h       |   4 +
>  migration/savevm.c    |   1 -
>  migration/savevm.h    |   2 +
>  qapi/migration.json   |   7 +-
>  8 files changed, 790 insertions(+), 12 deletions(-)
> 
> -- 
> 2.25.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK