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

[Eric Wheeler]

On Fri, 5 Dec 2014, Mike Snitzer wrote:
>> 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?

The additional overhead of worst-case should be (nearly) equal to the 
simplest test case of dd if=/dev/zero of=/dev/thinp/vol.  In my testing 
that was 1.4GB/s within KVM on an Intel Xeon(R) CPU E3-1230 V2 @ 3.30GHz.

I could see where really fast storage that can obtain 1.4gb/s might notice 
a performance regression---but for most use cases that is quite fast.

-Eric

> 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?




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