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disk performance strange/low

To: xfs@xxxxxxxxxxx
Subject: disk performance strange/low
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 17 Aug 2009 23:11:07 +0200
Organization: it-management http://it-management.at
User-agent: KMail/1.10.3 (Linux/2.6.30.4-ZMI; KDE/4.1.3; x86_64; ; )
I've got an Areca RAID Controller with 5x 1TB disks, and in the last 
time I often found it to be strange. Now I investigated, and found this. 
From another server I NFS mount a share, and make
dd if=/dev/sda of=/server/share/test.dd bs=1024k
so that should do big sequential writes. But the performance goes up and 
down, like you see in this output from "iostat -kx 5 555". You see write 
KB/s is 5824, 23207, 87283, 704, 40187. Up and down all the time. Could 
that be solved by the patch you discussed last time, or is there 
something else?

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-
sz avgqu-sz   await  svctm  %util
sdb               0,00   401,20    1,20   77,09     4,78  5824,00   
148,91   152,25 2595,77  12,76  99,92

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0,00    0,00   34,77   67,03    2,20    7,21
sdb               0,00  1094,40    2,20  245,80     8,80 23207,20   
187,23   153,64   43,24   4,03 100,00

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0,00    0,00   41,08   56,83    0,30   15,65
sdb               0,00   295,20    6,40  791,20    25,60 87283,40   
218,93   119,19  259,89   1,21  96,56

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0,00    0,00   61,45   55,92    0,50    4,62
sdb               0,00     3,60    0,00    4,60     0,00   704,80   
306,43   140,01 2979,30 217,39 100,00

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0,00    0,00   21,12   75,50    0,10   10,06
sdb               0,00  1252,80    0,20  270,00     0,80 40187,20   
297,47   151,38 1160,49   3,70  99,84


mfg zmi
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