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Old 07-08-2009, 04:43 PM   #1
Baffled
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Is sar & iostat output of svctm accurate


The svctm metric from my iostat and sar output is always sub-millisecond and I do not believe it. My storage array (HP EVA4400) is telling me that my READ latency is in the 12ms range. I am doing 90% READ activity. The iostat output for 2 of my devices looks like this (sorry about the wrapping):

DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
02:41:00 PM sde 1591.81 25146.44 180.41 15.91 23.85 14.99 0.63 99.58
02:41:00 PM sdac 1593.34 25155.90 171.75 15.90 22.29 13.99 0.62 99.57
02:41:15 PM sde 1549.00 24424.03 184.18 15.89 21.25 13.71 0.64 99.67
02:41:15 PM sdac 1570.76 24786.11 190.72 15.90 21.75 13.85 0.63 99.35
02:41:30 PM sde 1593.80 25146.27 169.80 15.88 22.50 14.12 0.62 99.48
02:41:30 PM sdac 1597.47 25176.00 198.73 15.88 22.32 13.97 0.62 99.21
02:41:45 PM sde 1603.46 25274.88 196.20 15.89 24.54 15.30 0.62 99.09
02:41:45 PM sdac 1599.00 25216.26 234.11 15.92 24.81 15.51 0.62 99.32


As you can see, the svctm reported here is less than 1ms.

I'm on RHEL5 U3

Thanks in advance

Last edited by Baffled; 07-08-2009 at 04:49 PM. Reason: correction: this is sar output, not iostat output
 
Old 07-09-2009, 02:15 PM   #2
Baffled
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sar device level -vs- Volume level metrics

My original post shows a subset of the sar output for 2 devices. Each entry in the sar output file also contains metrics for an entire Volume (rather than a device). These numbers, although more realistic, do not at all correlate to the sum of the individual devices. Here is output from one time slot:

01:20:59 PM DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
01:20:59 PM sde 1548.13 23120.00 1189.73 15.70 17.16 11.09 0.64 99.59
01:20:59 PM sdac 1567.73 23505.07 1038.53 15.66 17.76 11.33 0.64 99.57
01:20:59 PM sdr 1552.47 23126.40 1172.73 15.65 15.80 10.18 0.64 99.41
01:20:59 PM sdap 1545.20 23036.80 1119.53 15.63 14.84 9.61 0.64 99.43
01:20:59 PM sdg 1537.27 22822.40 1082.53 15.55 13.77 8.96 0.65 99.74
01:20:59 PM sdae 1575.20 23438.93 1088.33 15.57 14.85 9.43 0.63 99.65
01:20:59 PM sdt 1545.67 23134.93 1100.80 15.68 18.00 11.68 0.64 99.52
01:20:59 PM sdar 1553.47 23265.07 1048.53 15.65 18.37 11.85 0.64 99.31
01:20:59 PM dev199-65532 0.20 0.00 3.20 16.00 0.01 28.67 28.67 0.57
01:20:59 PM dev199-65531 0.27 0.00 4.27 16.00 0.00 0.00 0.00 0.00
01:20:59 PM dev199-65530 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Note that for the volume denoted by 'dev199-65532', the svctm is 28.67ms, however, each device is showing a svctm of approximately .64ms for this same time period. Can anyone explain the discrepancy?

Thank you
 
  


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