Re: [Qemu-devel] [PATCH v2 0/6] external backup api

["Denis V. Lunev" <den@virtuozzo.com>]

On 02/09/2016 07:49 PM, John Snow wrote:
>
> On 02/09/2016 09:37 AM, Denis V. Lunev wrote:
>> On 02/09/2016 05:21 PM, Stefan Hajnoczi wrote:
>>> On Fri, Feb 05, 2016 at 11:28:42AM +0300, Denis V. Lunev wrote:
>>>> On 02/03/2016 11:14 AM, Fam Zheng wrote:
>>>>> On Sat, 01/30 13:56, Vladimir Sementsov-Ogievskiy wrote:
>>>>>> Hi all.
>>>>>>
>>>>>> These series which aims to add external backup api. This is needed
>>>>>> to allow
>>>>>> backup software use our dirty bitmaps.
>>>>>>
>>>>>> Vmware and Parallels Cloud Server have this feature.
>>>>> What is the advantage of this appraoch over "drive-backup
>>>>> sync=incremental
>>>>> ..."?
>>>> This will allow third-party vendors to backup QEMU VMs into
>>>> their own formats or to the cloud etc.
>>> Backup software can implement NBD to receive the incremental blocks from
>>> QEMU.  Use drive-backup sync=incremental and the backup appliance as the
>>> NBD target.
>>>
>>> It's more complicated to add this QMP command flow to backup software
>>> than to implement NBD.
>>>
>>> Stefan
>> it can, but this is a matter of problem due to the nature of
>> how this software is implemented. Usually it is written
>> in a semi-standard way and it uses "plugins" to actually
>> collect the data, i.e. the code is written in standard
>> interface/real implementation pattern and interfaces are
>> basically the same.
>>
>> With this standard approach backup software is working
>> as an active side of the process, i.e. it performs operations
>> and controls the flow.
>>
>> This means that "non-standard" QEMU technology will be
>> pain here.
>>
>> Den
> Am I to understand that for e.g. VMWare the backup appliance is
> literally reading the disk image from storage directly while the VM is
> running?
>
> I'd be a bit surprised if that were true.
I think that backup software is asking alive VM about the data.

> My biggest concern here is that there is not a safe way, today, to read
> from a disk image atomically while the VM is running. I think that'd
> take a lot of work to do and you'll not find a lot of support in
> implementing it.
>
> Of course, while the VM is paused/off is a different story, but for now
> I still feel like NBD is the right answer for getting block data from QEMU.
>
> What am I missing?
>
> --js
In general, in Parallels Server the backup was created using the
following approach:
- create external snapshot. In this case the base image (backing store 
in QEMU terminology)
   will be READ-ONLY and could be safely read by any entity
- backup that read-only disk image (any way)
- merge snapshots

In this process backup software is active while PCS is passive.
With QEMU the approach looks the same to me:
- start a backup
- ask QEMU to give a data to be backuped (using NBD server in QEMU
   with Fam's image fleecing)
- finish backup

Important bit here is that dirty bitmap should be provided
by QEMU by request. This dirty bitmap will be read-only at that
moment, current active dirty bitmap should be replaced with
new at backup start operation.

Den