On 04/14/2014 06:40 AM, Nikolaus Jeremic wrote:
> Hello,
>
> how can I nudge fio to write a certain amount of data (less than
> capacity of block device) to randomly generated offsets covering the
> whole address space of a block device (e.g. MD RAID device)?
>
> Using the size parameter yields offsets from 0 to size (thus, not
> covering the whole address range). Using a time based job leads to the
> desired result, but I would like two write a defined amount of data. I
> have already tried this with fio 2.0.14, 2.1.6.1, 2.1.17, and 2.1.18
> without success. I am using Gentoo Linux with kernel 3.13.6 and libaio
> 0.3.109. The job description looks as follows:

Fio doesn't currently support that. Can you try the attached? It adds an 
option to separately define an io limit, io_limit. If io_limit is not 
set, size= behaves as it did before: if you don't give a size, size is 
defined by the files added. With this patch, if you do

filename=/dev/md9
size=40g

fio would read 40g between 0..40G

If you do:

filename=/dev/md9

fio would access the entire device.

If you do:

filename=/dev/md9
io_limit=10g

fio would access the entire device, quit when 10G of IO was done. Etc.

-- 
Jens Axboe