[dm-devel] dm thin: optimize away writing all zeroes to unprovisioned blocks

[Mike Snitzer]

On Fri, Dec 05 2014 at 12:27pm -0500,
Jens Axboe <axboe at kernel.dk> wrote:

> On 12/04/2014 08:33 AM, Mike Snitzer wrote:
> >
> >Here is a revised patch that is more like I'd expect to land upstream.
> >Jens are you OK with us adding bio_is_zero_filled to block/bio.c?  If so
> >should I split it out as a separate patch for you to pick up or just
> >carry it as part of the patch that lands in linux-dm.git?
> 
> Yeah I'm fine with adding that helper.

Good to know, thanks.

> I do wonder what the performance impact is on this for dm. Have you
> tried a (worst case) test of writing blocks that are zero filled, but
> with the last byte not being a zero?

I haven't tested that yet, but I share your concern.  Does fio offer the
ability to slap down a specific pattern like this?  If not is there a
logical place to extend fio to enable this?