Re: [PATCH v10 0/5] UFFD write-tracking migration/snapshots

From: Peter Xu <peterx@redhat.com>
To: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	qemu-devel@nongnu.org,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>, Den Lunev <den@openvz.org>
Subject: Re: [PATCH v10 0/5] UFFD write-tracking migration/snapshots
Date: Tue, 5 Jan 2021 14:36:58 -0500	[thread overview]
Message-ID: <20210105193658.GE149908@xz-x1> (raw)
In-Reply-To: <20201217165712.369061-1-andrey.gruzdev@virtuozzo.com>

On Thu, Dec 17, 2020 at 07:57:07PM +0300, Andrey Gruzdev wrote:
> 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.

For the rest patches:

Acked-by: Peter Xu <peterx@redhat.com>

Dave, considering the live snapshot series has been dangling for quite some
time upstream (starting from Denis's work), do you have plan to review/merge it
in the near future?

I believe there're still quite a few things missing, but imho most of them
should be doable on top too.

Thanks!

-- 
Peter Xu