Surprising calibrate_io result

Recently I used DBMS_RESOURCE_MANAGER.calibrate_io to measure disk I/O performance using a call like this:

DECLARE
 l_latency PLS_INTEGER;
 l_iops PLS_INTEGER;
 l_mbps PLS_INTEGER;
BEGIN

 DBMS_RESOURCE_MANAGER.calibrate_io (num_physical_disks => 10,
 max_latency => 20,
 max_iops => l_iops,
 max_mbps => l_mbps,
 actual_latency => l_latency);

 DBMS_OUTPUT.put_line('Max IOPS = ' || l_iops);
 DBMS_OUTPUT.put_line('Max MBPS = ' || l_mbps);
 DBMS_OUTPUT.put_line('Latency = ' || l_latency);

END;
/

Using this call I have a surprising result.  This test returns very different results when run against two different databases on the same Linux virtual machine and the same filesystem.

database 1:

Max IOPS = 7459
Max MBPS = 863
Latency = 18

database 2:

Max IOPS = 39921
Max MBPS = 1105
Latency = 0

Both databases use direct and asynchronous I/O.  The databases differ in size and configuration.  It seems that something about the databases themselves affects the results since they share the same filesystem on the same machine.

I did not get useful numbers from calibrate_io and have wasted a lot of time trying to interpret its results.  You may want to focus on other tools for measuring disk I/O performance.

– Bobby

About Bobby

I live in Chandler, Arizona with my wife and three daughters. I work for US Foods, the second largest food distribution company in the United States. I have worked in the Information Technology field since 1989. I have a passion for Oracle database performance tuning because I enjoy challenging technical problems that require an understanding of computer science. I enjoy communicating with people about my work.

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3 Responses to Surprising calibrate_io result

  1. Pingback: I/O Calibration Using DBMS_RESOURCE_MANAGER and RMAN Backups – Yet Another OCM

  2. John Hallas says:

    Do the two databases have a significantly different number of datafiles and is the overall size of both databases similar

    • Bobby says:

      John, the databases are pretty different. I’ve done the tests on the fast database with a few datafiles and with many and it is fast both ways. The slow database has autoextend on and maybe the datafiles extended many times. I have not had time to research it. Clearly something about the databases caused the different results which means the test doesn’t purely measure the performance of the I/O system. Thank you for your comment.

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