Monitoring the I/O Subsystem (Informix) (SAP Library

Monitoring the I/O Subsystem (Informix)

Use

You can use the SAP/Informix database monitor to monitor the Informix I/O subsystem.

In addition to the important question of when pages are written to disk, you can determine how many processes (that is, virtual processors or VPs) actually do the writing and how many threads (that is, page cleaner threads) should manage the I/O. Refer to Monitoring Checkpoints (Informix).

Procedure

Check for disk hotspots:

Choose Tools ® CCMS ® Control/Monitoring ® Performance Menu ® Database ® Activity ® Detail Analysis Menu ® Chunk I/O Activity.

Try to distribute I/O load over all disks that are available to the database.

For more information, see Checking Chunk I/O Activity (Informix).

Check the cleaners.

With the CLEANERS parameter in the Informix ONCONFIG file you specify how many threads do the work of organizing I/O. Here the general rule is to configure one cleaner thread per physical disk, with a rough maximum of three per controller. The maximum is 32 cleaner threads in total.

Check the asynchronous I/O virtual processors (AIOVPs).

For most current SAP hardware platforms, I/O is performed using separate AIO VPs. You configure the number of AIO VPs using the NUMAIOVPS parameter in the Informix ONCONFIG file.

Check the AIO.

Certain operating systems (for example, Sun Solaris) support kernel asynchronous I/O (KAIO). When the operating system allows it, this support is built into the Informix system (that is, you do not need to configure it). When present, the CPU VPs use KAIO threads to tell the operating system to perform the I/O asynchronously. There is no context switch to an AIO VP, and the CPU VP can continue processing without waiting for the I/O.

We suggest the following:

If KAIO is implemented, then make sure that NUMAIOVPS is set to 2.

If you are not using KAIO, you can find out whether you need to add AIO VPs as follows:

Use the global view of your system status

Choose Tools ® CCMS ® Control/Monitoring ® Performance Menu ® Database ® Activity ® Detail Analysis Menu ® Wait Situations.

If threads spend more time running than waiting on I/O, you probably do not have an I/O problem. For more information, see Checking Wait Situations (Informix).

Use onstat -g iov

This command shows you all I/O VPs. Look at those in the class AIO. Informix takes the first free AIO VP in order when I/O is performed. So if you see that the first AIO VPs in the list have performed most of the operations and the last AIO VPs relatively few operations, then you probably have enough AIO VPs.

The general rule is to set NUMAIOVPS = <number of disks> .

See also:

CLEANERS (Informix)

NUMAIOVPS (Informix)