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
- Parameter type
- 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
- 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.
number of prefetchers = max( max( max over all table spaces( parallelism setting ), number of cores * number of SMT threads ), 16 )
Recommendation: In order to fully use 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 more I/O servers, since there is some additional processing time that is associated with each I/O server and any unused I/O servers remain idle.