Question on --buffer_compress_percentage

Question on --buffer_compress_percentage

[Matthew Eaton]

Hello fio group,

Can anyone tell me if --buffer_compress_percentage fills the buffer
with random data and reuses that data or if it refills with random
data for each submit?  If you want the latter behavior should you
combine with --refill_buffers?

Thanks for your help!

-Matt

Re: Question on --buffer_compress_percentage

[Jens Axboe]

On Fri, May 24 2013, Matthew Eaton wrote:
> Hello fio group,
> 
> Can anyone tell me if --buffer_compress_percentage fills the buffer
> with random data and reuses that data or if it refills with random
> data for each submit?  If you want the latter behavior should you
> combine with --refill_buffers?

That is actually explained in the man page / HOWTO:

buffer_compress_percentage=int  If this is set, then fio will attempt to
                provide IO buffer content (on WRITEs) that compress to
                the specified level. Fio does this by providing a mix of
                random data and zeroes. Note that this is per block size
                unit, for file/disk wide compression level that matches
                this setting, you'll also want to set refill_buffers.

So the answer to your question is yes. Fio will reuse IO buffers, unless
--refill_buffers is set. If you do not set refill_buffers with
buffer_compress_percentage, then you will have identically compressible
blocks written. And that would only give you the per-block compression
ratio specified, on a broader scale it would compress more.

-- 
Jens Axboe

Re: Question on --buffer_compress_percentage

[Matthew Eaton]

Hi Jens,

Thank you for your explanation.  I have another question in regards to
buffer_compress_percentage if you don't mind.

If I were to call fio with "--refill_buffers
--buffer_compress_percentage=0", would this be the same as setting
only "--refill_buffers"?

Also, if I were to call fio with "--refill_buffers
--buffer_compress_percentage=100", would this be the same as setting
"--zero_buffers"?

Thanks again for your help!

On Mon, May 27, 2013 at 1:21 AM, Jens Axboe <axboe@kernel.dk> wrote:
> On Fri, May 24 2013, Matthew Eaton wrote:
>> Hello fio group,
>>
>> Can anyone tell me if --buffer_compress_percentage fills the buffer
>> with random data and reuses that data or if it refills with random
>> data for each submit?  If you want the latter behavior should you
>> combine with --refill_buffers?
>
> That is actually explained in the man page / HOWTO:
>
> buffer_compress_percentage=int  If this is set, then fio will attempt to
>                 provide IO buffer content (on WRITEs) that compress to
>                 the specified level. Fio does this by providing a mix of
>                 random data and zeroes. Note that this is per block size
>                 unit, for file/disk wide compression level that matches
>                 this setting, you'll also want to set refill_buffers.
>
> So the answer to your question is yes. Fio will reuse IO buffers, unless
> --refill_buffers is set. If you do not set refill_buffers with
> buffer_compress_percentage, then you will have identically compressible
> blocks written. And that would only give you the per-block compression
> ratio specified, on a broader scale it would compress more.
>
> --
> Jens Axboe
>

Re: Question on --buffer_compress_percentage

[Jens Axboe]

On Sat, Jun 01 2013, Matthew Eaton wrote:
> Hi Jens,
> 
> Thank you for your explanation.  I have another question in regards to
> buffer_compress_percentage if you don't mind.
> 
> If I were to call fio with "--refill_buffers
> --buffer_compress_percentage=0", would this be the same as setting
> only "--refill_buffers"?

Yes, those would be equivalant as the default is to use fully randomized
data contents.

> Also, if I were to call fio with "--refill_buffers
> --buffer_compress_percentage=100", would this be the same as setting
> "--zero_buffers"?

Yes.

-- 
Jens Axboe