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. 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
