Hi David,

Thanks for cc me, it was really exciting to know that write-protect feature finally been merged.
Exclude live memory snapshot, I'm thinking if we can use it to realize the real memory throttle in migration,
Since we still can come across dirty pages fail to converge with current cpu throttle method.
we may use write-protect capability to slow down the accessing speed of guest's memory, in order to
slow down the dirty pages ..., I'll look into it.

Besides, I'll follow this snapshot series, and to see if I can do some works to make this feature to be perfect enough
To be accepted as quickly as possible. ;)


Thanks,
Hailiang

> -----Original Message-----
> From: Dr. David Alan Gilbert [mailto:dgilbert@redhat.com]
> Sent: Tuesday, July 28, 2020 1:00 AM
> To: Denis Plotnikov <dplotnikov@virtuozzo.com>; david@redhat.com;
> Zhanghailiang <zhang.zhanghailiang@huawei.com>
> Cc: qemu-devel@nongnu.org; pbonzini@redhat.com; quintela@redhat.com;
> eblake@redhat.com; armbru@redhat.com; peterx@redhat.com;
> den@openvz.org
> Subject: Re: [PATCH v0 0/4] background snapshot
> 
> * 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