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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapsh

From: Denis V. Lunev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI
Date: Tue, 13 Feb 2018 18:01:17 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 02/13/2018 05:59 PM, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (address@hidden) wrote:
>> On Tue, Feb 13, 2018 at 03:45:21PM +0100, Kevin Wolf wrote:
>>> Am 13.02.2018 um 15:36 hat Daniel P. Berrangé geschrieben:
>>>> On Tue, Feb 13, 2018 at 05:30:02PM +0300, Roman Kagan wrote:
>>>>> On Tue, Feb 13, 2018 at 11:50:24AM +0100, Kevin Wolf wrote:
>>>>>> Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben:
>>>>>>> Then you could just use the regular migrate QMP commands for loading
>>>>>>> and saving snapshots.
>>>>>> Yes, you could. I think for a proper implementation you would want to do
>>>>>> better, though. Live migration provides just a stream, but that's not
>>>>>> really well suited for snapshots. When a RAM page is dirtied, you just
>>>>>> want to overwrite the old version of it in a snapshot [...]
>>>>> This means the point in time where the guest state is snapshotted is not
>>>>> when the command is issued, but any unpredictable amount of time later.
>>>>> I'm not sure this is what a user expects.
>>>>> A better approach for the save part appears to be to stop the vcpus,
>>>>> dump the device state, resume the vcpus, and save the memory contents in
>>>>> the background, prioritizing the old copies of the pages that change.
>>>>> No multiple copies of the same page would have to be saved so the stream
>>>>> format would be fine.  For the load part the usual inmigrate should
>>>>> work.
>>>> No, that's policy decision that doesn't matter from QMP pov. If the mgmt
>>>> app wants the snapshot to be wrt to the initial time, it can simply
>>>> invoke the "stop" QMP command before doing the live migration and
>>>> "cont" afterwards.
>>> That would be non-live. I think Roman means a live snapshot that saves
>>> the state at the beginning of the operation. Basically the difference
>>> between blockdev-backup (state at the beginning) and blockdev-mirror
>>> (state at the end), except for a whole VM.
>> That doesn't seem practical unless you can instantaneously write out
>> the entire guest RAM to disk without blocking, or can somehow snapshot
>> the RAM so you can write out a consistent view of the original RAM,
>> while the guest continues to dirty RAM pages.
> People have suggested doing something like that with userfault write
> mode; but the same would also be doable just by write protecting the
> whole of RAM and then following the faults.

nope, userfault fd does not help :( We have tried, the functionality is not
enough. Better to have small extension to KVM to protect all memory
and notify QEMU with accessed address.


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