Re: [Qemu-devel] [Qemu-block] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

On 13.02.2018 17:43, Kevin Wolf wrote:
> Am 13.02.2018 um 15:30 hat Roman Kagan geschrieben:
>> 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.
> 
> I don't think it's necessarily a big problem as long as you set the
> expectations right, but good point anyway.
> 
>> 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.
> 
> So basically you would let the guest fault whenever it writes to a page
> that is not saved yet, and then save it first before you make the page
> writable again? Essentially blockdev-backup, except for RAM.
> 
>> 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.
> 
> This is true.
> 
> I think it will require changes to the qcow2 driver, though. Currently,
> you write the VM state into the active layer and then take the disk
> snapshot so that the VM state is automatically snapshotted as well.
> Afterwards, the VM state is discarded again in the active layer.
> 
> If you want to take the snapshot at the point in time when the command
> was issued, you first need to create a qcow2 snapshot to save the disk
> state, but then continue to write the VM state into that snapshot, even
> though it's not the active layer.
> 
> In fact I think this would be more elegant than writing the VM state
> into the active layer and then discarding it again, but inactive
> snapshots haven't been writable so far, so that will require some
> changes.
> 
> I'm sure that Denis has already some thoughts on this, though.
Yes, I stick to the scheme when you write a snapshot as it is now but 
instead of ram writing I just preallocate the space for ram and save the 
actual file offsets of the clusters forming the space, then I resume 
vcpus and write the ram using those cluster offsets with respect to vcpu 
pagefaults.

> 
> Kevin
> 

-- 
Best,
Denis