Resolving The Problem

INTRODUCTION

You may need to monitor the disk performance of a system in order to make sure that there are no performance issues at a disk device level which could be affecting the performance of the IBM® Informix® Dynamic Server™ (IDS) database server. You can use the Unix dd command to monitor the reading performance of a disk device.


STEPS

Try three different tests. Try 100Mb, 1Gb and 2Gb tests. These tests should be run against raw devices so that the UNIX® file system buffer will not affect the measurements. These tests should also be done when there is no IDS activity (or any other) that will utilize the same devices.


Syntax:

dd if=input_file of=ouput_file bs=block_size count=number_of_blocks

input_file
The name of the input file you want dd the read from. You have to create a file in you file system with at least the size of the dd test you want to perform.
ouput_file
The name of the output file you want dd write the input_file to. Choose a device that is not being used by IDS.
block_size
The size of the block you want dd to use. (Use 2k for most IBM Informix systems, 4k for AIX and Windows).
count
The number of blocks you want dd to read.

timex dd if=/dev/rdsk/c0t2d0s1 of=/dev/null bs=2k count=50000
50000+0 records in
50000+0 records out

real       40.67
user        0.40

timex dd if=/dev/rdsk/c0t2d0s1 of=/dev/null bs=2k count=500000
500000+0 records in
500000+0 records out

real     6:45.69
user        2.31
sys        20.68

timex dd if=/dev/rdsk/c0t2d0s1 of=/dev/null bs=2k count=1000000
1000000+0 records in
1000000+0 records out

real    13:38.74
user        5.04
sys        39.23

Analyze the data:

The first test - Divide 100Mb by 40 seconds and you will have 2.5Mb/s.

The second test - Divide 1Gb by 444 seconds and you will have 2.5Mb/s.

The third test - Divide 2Gb by 818 seconds and you will have 2.4Mb/s.

The average reading throughput for the three tests is 2.5Mb/s.


Compare these numbers with all your devices to make sure you do not have a bad disk or a configuration problem. You should also compare your numbers against the specifications of the device.