Re: Amount of data read with mixed workload sequential/random with percentage_random set

[Juergen Salk <juergen.salk@uni-ulm.de>]

* Juergen Salk <juergen.salk@uni-ulm.de> [130918 16:58]:
> 
> --- snip ---
> 
> [global]
> ioengine=sync
> direct=0
> # Block sizes for I/O units: 25% 19k, 15% 177k, 60% 350k 
> bssplit=19k/25:177k/15:350k/60
> # Use mixed workload: 30% random IO, 70% sequential IO
> percentage_random=30
> size=${SIZE}
> numjobs=${NUMJOBS}
> runtime=${RUNTIME}
> directory=${DIRECTORY}
> 
> [application]
> # Define of I/O pattern: Random read. 
> rw=randread
> 
> --- snip ---
> 
> This is run with the following command: 
> 
> $ RUNTIME=0 NUMJOBS=4 SIZE=4096m DIRECTORY=/work/testsoft fio jobfile.fio >fio.out 2>&1
> 
> I have noticed from the output file, that this results in different 
> amounts of data read by the individual processes:
> 
> $ grep io= fio.out
> read : io=5847.5MB, bw=149458KB/s, iops=627, runt= 40063msec
> read : io=4096.2MB, bw=140358KB/s, iops=595, runt= 29884msec
> read : io=4096.3MB, bw=140889KB/s, iops=596, runt= 29772msec
> read : io=5246.4MB, bw=134821KB/s, iops=560, runt= 39847msec
>  READ: io=19286MB, aggrb=492947KB/s, minb=134820KB/s, maxb=149458KB/s, mint=29772msec, maxt=40063msec 
> 
> I have expected that every individual process will read 
> its 4096 MB and then stop further reading. Or am I missing 
> something?

Hi,

I'm still a bit puzzled about the amount of data read by
individual processes spawned by fio. Given the following (now
simplified) job file:

--- snip ---
[global]
ioengine=sync
direct=0
bssplit=19k/25:177k/15:350k/60
size=100m
numjobs=4
directory=/tmp

[work]
rw=randread
--- snip ---

$ fio jobfile.fio >fio.out
$ grep io= fio.out
  read : io=199968KB, bw=4892.6KB/s, iops=27, runt= 40872msec
  read : io=200062KB, bw=5083.5KB/s, iops=28, runt= 39359msec
  read : io=200156KB, bw=4989.1KB/s, iops=27, runt= 40112msec
  read : io=199940KB, bw=4492.4KB/s, iops=24, runt= 44507msec
   READ: io=800126KB, aggrb=17977KB/s, minb=4492KB/s, maxb=5083KB/s, mint=39359msec, maxt=44507msec

I.e. every individual process reads approx. 200 MB of data rather 
than 100 MB as specified in the job file. For sequential reads 
(i.e. replaced rw=randread by rw=read, but otherwise unchanged job 
file) the amount of data read by each process is close to 100 MB as 
expected.

I am probably missing something obvious, but why does the job file 
above result in 200 MB read by every process?

Thanks in advance.

Juergen