Prefetch Hits in Performance Monitor

Products

• IDMS

Issue/Introduction

The Buffer I/O Summary screen in Performance Monitor has a field called Prefetch Hits. What exactly does this statistic represent?

Environment

Release: All supported releases.

Resolution

When IDMS loads a database page into a buffer, it sets a flag for that page to record whether or not it was loaded by the prefetch process. Whenever that page is subsequently requested by IDMS, it will find it in the buffer, and then check that bit. If it is on, the Prefetch Hits value is incremented.

So Prefetch Hits is a measure of how many of the "Fnd_In Buffer" number were there because the page had been loaded by prefetch.

Additional Information

Using Chained Reads
PREFETCH: how it is enabled/disabled and when to use it