Re: [Qemu-devel] [PATCH] block: don't probe zeroes in bs->file by default on block_status

[Kevin Wolf <kwolf@redhat.com>]

Am 10.01.2019 um 14:20 hat Vladimir Sementsov-Ogievskiy geschrieben:
> drv_co_block_status digs bs->file for additional, more accurate search
> for hole inside region, reported as DATA by bs since 5daa74a6ebc.
> 
> This accuracy is not free: assume we have qcow2 disk. Actually, qcow2
> knows, where are holes and where is data. But every block_status
> request calls lseek additionally. Assume a big disk, full of
> data, in any iterative copying block job (or img convert) we'll call
> lseek(HOLE) on every iteration, and each of these lseeks will have to
> iterate through all metadata up to the end of file. It's obviously
> ineffective behavior. And for many scenarios we don't need this lseek
> at all.
> 
> So, let's "5daa74a6ebc" by default, leaving an option to return
> previous behavior, which is needed for scenarios with preallocated
> images.
> 
> Add iotest illustrating new option semantics.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

I still think that an option isn't a good solution and we should try use
some heuristics instead.

Kevin