Tuning Your Applications › Data Delays

Data Delays

This section describes the various types of data delays, and the possible reasons for them.

Placement of data sets

Queueing delays are occurring because data sets used by the program or one of its subroutines reside on busy DASD volumes.

Insufficient buffers for sequential access methods

Too few buffers are defined for a file to provide timely sequential data set access processing.

Block size too small for sequential access methods

Indicates one of the following reasons:

The block size is too small and causing too many I/O operations.

Delays are reported for both the processing time to start and to complete the I/O operation and for the wait for data to be returned.

File is fragmented into many extents

Indicates one of the following reasons:

The file used by the program or one of its subroutines is in many extents.

This fragmentation causes additional seeking to take place on the device, which delays the program.

VSAM files poorly buffered

An insufficient number of index and data buffers are defined for VSAM files, or the buffers are too small.

VSAM files use share options (4,4)

Indicates one of the following reasons:

When VSAM data sets must be open in more than one address space for updating, each application must use share options that preserve the integrity of the data

When these share options are used, each read requires an I/O operation to refresh the data because another address space might have updated the data.

Insufficient buffers for VSAM files that use local or global shared resources

Indicates one of the following reasons:

If a program uses VSAM shared resources for multiple files to better utilize virtual memory, not having enough buffers of a required size causes delays waiting for buffers to become available.

This situation is especially important for read operations because the intent is to find the data in the memory buffer as often as possible.

Reduce data delays by tuning the buffers to the workload that uses the buffer pool.

DB2 SQL process time

Indicates one of the following reasons:

An application might spend an inordinate amount of time waiting for data to be returned for an SQL statement.

This problem can be caused by excessive page scans if an INDEX is not available to speed the data retrieval. For more information about SQL delays, see the chapter "Using the Product in a DB2 Environment."

IMS database process time

An application might spend inordinate amounts of time waiting for data from an IMS database to be returned.

For more information about IMS delays, see the chapter "Using the Product in an IMS Environment."