Re: [PATCH 0/4] qemu-img: Fix exit code for errors closing the image

[Vladimir Sementsov-Ogievskiy]

On 13.01.23 14:29, Kevin Wolf wrote:
> Another thing that could be tried is making failure in .bdrv_close less
> likely by doing things earlier. At least ENOSPC could probably be
> avoided if dirty bitmaps clusters were allocated during the write
> request that first sets a bit in them (I know too little about the
> details how bitmaps are implemented in qcow2, though, maybe Vladimir can
> help here).

That's possible but not trivial :)

Qcow2 does nothing with dirty bitmaps during normal operation. Only on close, 
it finds all persistent bitmaps and stores them somehow, mostly allocating new 
clusters on the fly.

So the simplest way look like:

- add generic handler .bitmap_changed in BlockDriver, to handle bitmap change 
in qcow2 (that's not only write, but may be bitmap_merge opertion).
- in a new handler allocate some clusters to produce a pool for dirty bitmaps 
saving (will need clusters for bitmap data and metadata (bitmap table, bitmap 
directory))
- in block/qcow2-bitmap.c switch qcow2_alloc_cluster() to a wrapper, that first 
tries to get a cluster from the pool and if it's empty fallback to 
qcow2_alloc_cluster()

Note also, that this will increase fragmentation.

Or, may be more effective would be to preallocate clusters on bitmap creation 
(and therefore on image resize).

More difficult would be rework the whole code to bind allocated clusters for 
each persistent dirty bitmap.

--
Best regards,
Vladimir