parallel prefetching

For situations in which multiple containers exist for a table space, the database manager instance can initiate parallel I/O. Parallel I/O refers to the ability to use multiple I/O servers to process the I/O requirements of a single query. Each I/O server is assigned the I/O workload for a separate container, allowing several containers to be read in parallel. Performing I/O in parallel can result in significant improvements to I/O throughput.

While a separate I/O server will handle the workload for each container, the actual number of I/O servers that can perform I/O in parallel will be limited to the number of physical devices over which the requested data is spread. This also means you need as many I/O servers as the number of physical devices.

How parallel I/O is initiated and used is dependent on the reason for performing the I/O: for sequential prefetch, parallel I/O is initiated when the prefetch size is a multiple of the extent size for a table space. Each prefetch request is then broken into multiple, smaller, requests along the extent boundaries. These smaller requests are then assigned to different I/O servers.