DB2 Version 10.1 for Linux, UNIX, and Windows

num_ioservers - Number of I/O servers configuration parameter

This parameter specifies the number of I/O servers for a database. No more than this number of I/Os for prefetching and utilities can be in progress for a database at any time.

Configuration type

Database

Parameter type

Configurable
Configurable by member in a DB2® pureScale® environment

Default [range]

Automatic [1 - 255 ]

Note: The default value is subject to change by the DB2 Configuration Advisor after initial database creation.

Unit of measure

Counter

When allocated

When an application connects to a database

When freed

When an application disconnects from a database

I/O servers, also called prefetchers, are used on behalf of the database agents to perform prefetch I/O and asynchronous I/O by utilities such as backup and restore. An I/O server waits while an I/O operation that it initiated is in progress. Non-prefetch I/Os are scheduled directly from the database agents and as a result are not constrained by num_ioservers.

If this parameter is set to AUTOMATIC, the number of prefetchers started will be based on the parallelism settings of the table spaces in the current database partition.

When this parameter is set to AUTOMATIC, the number of prefetchers to start will be calculated at database activation time based on the following formula:

number of prefetchers = max( max over all table spaces( parallelism setting ), 3 )

Where parallelism settings are controlled by the DB2_PARALLEL_IO system environment variable.

Recommendation: In order to fully exploit all the I/O devices in the system, a good value to use is generally one or two more than the number of physical devices on which the database resides. It is better to configure additional I/O servers, since there is some additional processing time associated with each I/O server and any unused I/O servers will remain idle.