Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots

From: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
To: Peter Krempa <pkrempa@redhat.com>
Cc: qemu-devel@nongnu.org, Den Lunev <den@openvz.org>,
	Eric Blake <eblake@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots
Date: Fri, 27 Nov 2020 13:00:48 +0300	[thread overview]
Message-ID: <23cbb153-9260-3357-04ba-94da7da8c0d2@virtuozzo.com> (raw)
In-Reply-To: <20201127094911.GI2366@angien.pipo.sk>

On 27.11.2020 12:49, Peter Krempa wrote:
> On Fri, Nov 27, 2020 at 11:21:39 +0300, Andrey Gruzdev wrote:
>> On 26.11.2020 18:47, Peter Krempa wrote:
>>> On Thu, Nov 26, 2020 at 18:17:28 +0300, Andrey Gruzdev via 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.
>>>
>>> This sounds amazing! Based on your description I assume that the final
>>> memory image contains state image from the beginning of the migration.
>>>
>>> This would make it desirable for the 'migrate' qmp command to be used as
>>> part of a 'transaction' qmp command so that we can do an instant disk
>>> and memory snapshot without any extraneous pausing of the VM.
>>>
>>> I'll have a look at using this mechanism in libvirt natively.
>>>
>>
>> Correct, the final image contains state at the beginning of migration.
>>
>> So far, if I'm not missing something about libvirt, for external snapshot
>> creation it performs a sequence like that:
>> migrate(fd)->transaction(blockdev-snapshot-all)->cont.
>>
>> So, in case 'background-snapshot' capability is enabled, the sequence would
>> change to:
>> stop->transaction(blockdev-snapshot-all)->migrate(fd).
>> With background snapshot migration it will finish with VM running so there's
>> not need to 'cont' here.
> 
> Yes, that's correct.
> 
> The reason I've suggested that 'migrate' being part of a 'transaction'
> is that it would remove the need to stop it for the disk snapshot part.
> 

Hmm, I believe stopping VM for a short time is unavoidable to keep saved 
device state consistent with blockdev snapshot base.. May be I've missed 
something but it seems logical.

-- 
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                 virtuzzo.com