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

From: Andrey Gruzdev
Subject: Re: [PATCH v10 0/5] UFFD write-tracking migration/snapshots
Date: Wed, 6 Jan 2021 18:19:59 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 05.01.2021 22:36, Peter Xu wrote:
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.
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!
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397

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