Re: Question on --buffer_compress_percentage

[Jens Axboe <axboe@kernel.dk>]

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