Command Definition/General
Applicable release versions: AP/Unix, AP 6.2
Category General (155)
Description describes the mechanism by which a Pick process does blocked IO, in order to improve disk performance.
Processes doing sequential disk accesses, like the file save, or Access statements, have the possibility of grouping several disk reads into one larger disk access. This has the advantage of reducing the number of disk accesses and, therefore, improve performance. For example, selecting a 10,000 frame file normally requires 10,000 disk accesses. If the process can read four frames at a time, it will do only 2,500 disk accesses. Even if reading 4 frames (8 kilobytes on a 2 K frame system) takes a little bit longer than reading only 1 frame (2 kilobytes), reducing the number of disk accesses usually results in a noticeable performance improvement.

The cost of using blocked IO is a greater memory demand on the system, and might have some impact on multiuser performance.

The net effect of grouping the disk reads is reducing the number of 'frame faults' (i.e., when a Pick process requires a page which is not currently in memory). This can be monitored by the TCL command "buffers".

The blocking factor (or number of frames read in one disk access) can be set by the TCL command "blkio" for the whole system, or as an argument in the configuration file.
Related tcl.blkio