Performance Tuning WebWizard

Expanation of Disks

The disk are the simplest part of a computer but are unfortunely machanical. In modern computer systems the disks are inexpensive and so is often the first thing to be upgraded.

The disks are used for:

files filesystems including programs
paging memory
raw disks - typically a database.

Disks are connected to the machine via adapters:

SCSI - simple and cheap but only a limited number of disk can be connected to each adpater. For performance lower numbers of disk have to be used.
SSA - faster and capable of dozens of disks on a single adapter

Both disks and adapter can become a bottleneck although if sensable numbers of disks are attached to each adpater they rarely become the problem.

Disks have three performance figures:

seek time - the average time to move to the next place on the disk
transfer time - the time to read or write data
thoughput - the realistic time to read data because we sometimes have to wait to the rotating disk to move to the right bit of data.

Disk Bottleneck ???

Criteria:

Disk I/O bound when CPU %iowait greater than 40%
Disk bound when a disk is regularly more than 45% busy

In a multi-user environment disk usage is always likely to be very peaky - meaning that users demands of the disks are in lots of small unpredictable bursts. To cater for this we need to keep the disks less than 45% busy (some say 35% busy), so that 2 out of 3 times the disk is not busy and so can immediate action the next I/O request.

Disk nmon Statistics

The basic Disk usage is on the Main Screen of nmon but its much better to use the Disk Screen.

The nmon statisics comprises every thing you need in a simple format:

read kB/s Means the amount of data read from the disk in k bytes per second.
write kB/s Means the amount of data written to the disk in k bytes per second.
%busy Means the percentage of the time that the disk was active.
reads/s Means the number of read transfers per second.
writes/s Means the number of write transfers per second.

Notes:

Find more information about the size and type of disks with lsdev -Cc disk
For SSA disks also use lsdev -Cc pdisk.
AIX tends to use disk pages of 4k bytes (4096 bytes).

What can you do to disks?

If you find the disks are 100% busy:

Spread the I/O across more disks - particularly the "hot spots". With AIX this can do done online using the Logical Volume Manager. Using smitty and the migratelv command but you may need to also make database changes.
If CPU wiat is high but no one disk looks very busy it might be the adpater being over loaded.
Add memory to reduce the repeating reading of disk data.
Most disk problems can be avoided with careful planning by those that understand their database and workloads.
Plan for an upgrade!!

read kB/s	Means the amount of data read from the disk in k bytes per second.
write kB/s	Means the amount of data written to the disk in k bytes per second.
%busy	Means the percentage of the time that the disk was active.
reads/s	Means the number of read transfers per second.
writes/s	Means the number of write transfers per second.