DB2 Buffer Pool Monitoring

After setting up your buffer pools, you will want to regularly monitor your configuration for performance. The most rudimentary way of doing this is using the -DISPLAY BUFFERPOOL command. There are many options of the DISPLAY command that can be used to show different characteristics of your buffer pool environment; the simplest is the summary report, requested as follows:

-DISPLAY BUFFERPOOL(BP0) LIST(*) DBNAME(DSN8*)

And a truncated version of the results will look something like this:

DSNB401I - BUFFERPOOL NAME BP0, BUFFERPOOL ID 0, USE COUNT 20

DSNB402I - VIRTUAL BUFFERPOOL SIZE = 500 BUFFERS 736

             ALLOCATED = 500 TO BE DELETED = 0

             IN-USE/UPDATED = 0

DSNB404I - THRESHOLDS - 739

             VP SEQUENTIAL        = 80   HP SEQUENTIAL = 75

             DEFERRED WRITE       = 85   VERTICAL DEFERRED WRT = 80,0

             PARALLEL SEQUENTIAL  = 50   ASSISTING PARALLEL SEQT = 0

Of course, you can request much more information to be displayed using the DISPLAY BUFFERPOOL command by using the DETAIL parameter. Additionally, you can request that DB2 return either incremental statistics (since the last DISPLAY) or cumulative statistics (since DB2 was started). The statistics in a detail report are grouped in the following categories: GETPAGE information, Sequential Prefetch information, List Prefetch information, Dynamic Prefetch information, Page Update statistics, Page Write statistics, and Parallel Processing Activity details.

A lot of interesting and useful details can be gathered simply using the DISPLAY BUFFERPOOL command. For example, you can review GETPAGE requests for random and sequential activity, number of prefetch requests (whether static or dynamic, or for sequential or list prefetch), number of times each of the thresholds were tripped, and much more. Refer to the DB2 Command Reference manual (SC19-4054-02 for DB2 11) for a definition of each of the actual statistics returned by DISPLAY BUFFERPOOL.

Many organizations also have a performance monitor (such as IBM’s Omegamon) that simplifies the gathering and display of buffer pool statistics. Such tools are highly recommended for in-depth buffer pool monitoring and tuning. More sophisticated tools also exist that offer guidance on how to tune your buffer pools — or that automatically adjust your buffer pool parameters according to your workload. Most monitors also provide more in-depth statistics, such as buffer pool hit ratio calculations.

The buffer pool hit ratio is an important tool for monitoring and tuning your DB2 buffer pools. It is calculated as follows:

Hit ratio = GETPAGES - pages_read_from_disk / GETPAGEs

“Pages read from disk” is a calculated field that is the sum of all random and sequential reads.

The highest possible buffer pool hit ratio is 1.0. This value is achieved when each requested page is always in the buffer pool. When requested pages are not in the buffer pool, the hit ratio will be lower. You can have a negative hit ratio — this just means that prefetch was requested to bring pages into the buffer pool that were never actually referenced.

In general, the higher the hit ratio the better because it indicates that pages are being referenced from memory in the buffer pool more often. Of course, a low hit ratio is not always bad. The larger the amount of data that must be accessed by the application, the lower the hit ratio will tend to be. Hit ratios should be monitored in the context of the applications assigned to the buffer pool and should be compared against hit ratios from prior processing periods. Fluctuation can indicate problems.