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

[John Snow]

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.

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