Re: [Qemu-devel] [Qemu-block] [PATCH] file-posix: add rough-block-status parameter

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jan 08, 2019 at 10:45:52PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> bdrv_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, add an option to omit calling lseek.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all!
> 
> This is a continuation of "do not lseek in qcow2 block_status" thread
> (https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05749.html)

I don't have time to participate in the discussion so my (uneducated?)
opinion doesn't count for much, but I wish it were possible to perform
this optimization automatically without introducing a new user-visible
option.

That way existing users benefit and new users don't have to learn about
a new option.

Stefan

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