Re: Where is the performance bottleneck?

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Holger Kiehl (Holger.Kiehl@dwd.de) wrote:

> There is however one difference, here I had set
> /sys/block/sd?/queue/nr_requests to 4096.

Well from that it looks like none of the queues get about 255
(hmm that's a round number....)

> avg-cpu:  %user   %nice    %sys %iowait   %idle
>            0.10    0.00   21.85   58.55   19.50

Fair amount of system time.

> Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s 
> avgrq-sz avgqu-sz   await  svctm  %util

> sdf        11314.90   0.00 365.10  0.00 93440.00    0.00 46720.00     0.00  
> 255.93     1.92    5.26   2.74  99.98
> sdg        7973.20   0.00 257.20  0.00 65843.20    0.00 32921.60     0.00   
> 256.00     1.94    7.53   3.89 100.01

There seems to be quite a spread of read performance accross the drives
(pretty consistent accross the run); what makes sdg so much slower than
sdf (which seems to be the slowest and fastest drives respectively).
I guess if everyone was running at sdf's speed you would be pretty happy.

If you physically swap f and g does the performance follow the drive
or the letter?

Dave
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