Re: [Qemu-devel] [PATCH RFC] external backup api

[John Snow <jsnow@redhat.com>]

On 01/22/2016 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
> Hi all.
> 
> This is the early begin of the 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.
> 

Have a link to the equivalent feature that VMWare exposes? (Or Parallels
Cloud Server) ... I'm curious about what the API there looks like.

> There is only one patch here, about querying dirty bitmap from qemu by
> qmp command. It is just an updated and clipped (hmp command removed) old
> my patch "[PATCH RFC v3 01/14] qmp: add query-block-dirty-bitmap".
> 
> Before writing the whole thing I'd like to discuss the details. Or, may
> be there are existing plans on this topic, or may be someone already
> works on it?
> 
> I see it like this:
> 
> =====
> 
> - add qmp commands for dirty-bitmap functions: create_successor, abdicate,
> reclaime.

Hm, why do we need such low-level control over splitting and merging
bitmaps from an external client?

> - make create-successor command transaction-able
> - add query-block-dirty-bitmap qmp command
> 
> then, external backup:
> 
> qmp transaction {
>     external-snapshot
>     bitmap-create-successor
> }
> 
> qmp query frozen bitmap, not acquiring aio context.
> 
> do external backup, using snapshot and bitmap
> 
> if (success backup)
>     qmp bitmap-abdicate
> else
>     qmp bitmap-reclaime
> 
> qmp merge snapshot
> =====
> 

Hm, I see -- so you're hoping to manage the backup *entirely*
externally, so you want to be able to reach inside of QEMU and control
some status conditions to guarantee it'll be safe.

I'm not convinced QEMU can guarantee such things -- due to various flush
properties, race conditions on write, etc. QEMU handles all of this
internally in a non-public way at the moment.

> 
> In the following patch query-bitmap acquires aio context. This must be
> ofcourse dropped for frozen bitmap.
> But to make it in true way, I think, I should check somehow that this is
> not just frozen bitmap, but the bitmap frozen by qmp command, to avoid
> incorrect quering of bitmap frozen by internal backup (or other
> mechanizm).. May be, it is not necessary.
> 
> 
>