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

[Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>]

On 27.11.2020 18:45, Peter Xu wrote:
> On Fri, Nov 27, 2020 at 01:00:48PM +0300, Andrey Gruzdev wrote:
>> 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.
> 
> I guess PeterK meant an explicit stop vm command, rather than the stop in this
> series that should be undetectable from the guest.  It would be definitely
> great if PeterK could quickly follow up with this on libvirt. :)
> 

Sorry if I misunderstood, sure it would be great if PeterK is really 
interested to follow up with this feature!

> One overall comment (before go into details) is that please remember to start
> using "migration: " prefixes in patch subjects from the next version.
> 

Yes, I really missed it everywhere :)

> Thanks,
> 


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