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Will
Asked: 2014-04-25 14:44:58 +0800 CST

High iowait despite small amount of write/read

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I'm not sure how to interpret this or the next steps, but why is IOWait so high when I have not written/read much from any devices?

Background: I am a developer trying to find why the DB seems to be slow on a particular environment.

iostat -tm 60 15 Time: 05:17:01 PM avg-cpu: %user %nice %system %iowait %steal %idle 2.43 0.00 2.28 16.50 0.00 78.78 Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn cciss/c0d0 2.14 0.00 0.03 70614 859153 cciss/c0d0p1 0.00 0.00 0.00 41 2 cciss/c0d0p2 2.14 0.00 0.03 70572 859151 dm-0 2.51 0.00 0.01 19375 296915 dm-1 1.69 0.00 0.02 38433 548545 dm-2 0.22 0.00 0.00 12764 13689

Time: 05:18:01 PM avg-cpu: %user %nice %system %iowait %steal %idle 4.84 0.00 5.21 54.52 0.00 35.43 Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn cciss/c0d0 1.78 0.00 0.02 0 1 cciss/c0d0p1 0.00 0.00 0.00 0 0 cciss/c0d0p2 1.78 0.00 0.02 0 1 dm-0 2.47 0.00 0.01 0 0 dm-1 1.22 0.00 0.01 0 0 dm-2 0.00 0.00 0.00 0 0

Time: 05:19:01 PM avg-cpu: %user %nice %system %iowait %steal %idle 5.71 0.00 5.24 58.33 0.00 30.72 Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn cciss/c0d0 1.68 0.00 0.02 0 1 cciss/c0d0p1 0.00 0.00 0.00 0 0 cciss/c0d0p2 1.68 0.00 0.02 0 1 dm-0 1.88 0.00 0.01 0 0 dm-1 1.18 0.00 0.01 0 0 dm-2 0.00 0.00 0.00 0 0

linux

2 Answers

  1. You can run awr reports with the following. Oracle will tell you what it is waiting on in under "Top 5 Timed Events"

    -- run awr (timed_statistics=TYPICAL and control_management_pack_access='DIAGNOSTIC+TUNING')
connect / as sysdba
@$ORACLE_HOME/rdbms/admin/awrrpt.sql
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  2. You're seeing high iowait because the system is idle. Basically, the system has nothing to do but wait for I/O, so that's what it does. When there is lots of idle time, high iowait is not a problem.

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