Re: [PATCH 1/2] block/raw: added support of persistent dirty bitmaps

[Vladimir Sementsov-Ogievskiy]

20.03.2021 12:32, Patrik Janoušek wrote:
> Current implementation of dirty bitmaps for raw format is very
> limited, because those bitmaps cannot be persistent. Basically it
> makes sense, because the raw format doesn't have space where could
> be dirty bitmap stored when QEMU is offline. This patch solves it
> by storing content of every dirty bitmap in separate file on the
> host filesystem.
>
> However, this only solves one part of the problem. We also have to
> store information about the existence of the dirty bitmap. This is
> solved by adding custom options, that stores all required metadata
> about dirty bitmap (filename where is the bitmap stored on the
> host filesystem, granularity, persistence, etc.).
>
> Signed-off-by: Patrik Janoušek<pj@patrikjanousek.cz>


Hmm. Did you considered other ways? Honestly, I don't see a reason for yet 
another storing format for bitmaps.

The task could be simply solved with existing features:

1. We have extenal-data-file feature in qcow2 (read docs/interop/qcow2.txt). 
With this thing enabled, qcow2 file contains only metadata (persistent bitmaps 
for example) and data is stored in separate sequential raw file. I think you 
should start from it.

2. If for some reason [1] doesn't work for you, you can anyway use an empty qcow2 file to 
store bitmaps instead of inventing and implementing new format of bitmaps storing. (Same 
as your approach, you'll have a simple raw node, and additional options will say 
"load bitmaps from this qcow2 file". But for such options we'll need good 
reasons why [1] isn't enough.

--
Best regards,
Vladimir