Calculation of stdev

Hey, I'm back with another note ;)

I've been running the following job :

job1: (groupid=0, jobs=1): err= 0: pid=9099
  Description : ["Read Seq 16k O/Direct"]
  read : io=16384KiB, bw=13315KiB/s, iops=812, runt= 1260msec
    clat (usec): min= 278, max=317965, avg=1225.76, stdev=13734.77
    bw (KiB/s) : min= 3996, max= 3996, per=30.01%, avg=3996.00, stdev=-1.00
  cpu : usr=0.00%, sys=5.40%, ctx=1029
  IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%,
>=64=0.0%
     issued r/w: total=1024/0, short=0/0
     lat (usec): 500=91.31%, 750=2.54%, 1000=0.10% lat (msec):
2=2.44%, 4=3.03%, 10=0.20%, 20=0.10%, 100=0.10% lat (msec): 500=0.20%

I'm really surprised that stdev for bw is set to -1 whereas the min, max
& average are equal.
Stdev in such situation should be 0 isn't it ?
Received on Thu Jul 19 2007 - 17:34:52 CEST