Re: [Qemu-devel] [PATCH 1/4] block: Fix dirty bitmap in bdrv_co_discard

[Paolo Bonzini]

On 11/05/2015 10:02, Fam Zheng wrote:
> 
>       /*
>        * ...
>        *
>        * 'pnum' is set to the number of sectors (including and immediately 
> following
>        * the specified sector) that are known to be in the same
>        * allocated/unallocated state.
>        *
>        * 'nb_sectors' is the max value 'pnum' should be set to.  If 
> nb_sectors goes
>        * beyond the end of the disk image it will be clamped.
>        */
>       static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState 
> *bs,
> 
> So we currently don't coalesce the sequential dirty sectors.
> 
> Was that intentional?

The idea, I think, is that for some drivers bdrv_co_get_block_status is
O(nb_sectors).

It's okay to let a driver return *pnum > nb_sectors.  You do need to
audit the callers, though.  It would be nice also to add TODOs whenever
the code is fine but it could explot *pnum > nb_sectors to get better
performance.

Paolo