Re: dm thin: optimize away writing all zeroes to unprovisioned blocks

[Jens Axboe <axboe@kernel.dk>]

On 12/05/2014 11:33 AM, Mike Snitzer wrote:
> On Fri, Dec 05 2014 at 12:27pm -0500,
> Jens Axboe <axboe@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?

You can do an explicit pattern for IO buffers, but it's not really 
geared towards this, it's more for repeat patterns. But maybe something 
could be added, let me think about the best way to do that...

-- 
Jens Axboe