When you are using the prefetch facility to input multiple pages at a time into a global buffer, pages are synchronously pre-fetched from the DB process and input to the prefetch buffer. When the prefetch facility is being used, the asynchronous READ facility sets up two prefetch buffers and, while the DB process is using one of the buffers, a READ process pre-fetches pages from the other buffer asynchronously with the DB process. By executing the DB processing and the prefetch input at the same time, you can reduce the processing time. Moreover, with HiRDB/Parallel Server, by alternating the threads for the I/O wait time, you can reduce the I/O wait time.

Note, however, that you cannot use the asynchronous READ facility with a local buffer. Nor can you use it on an RDAREA that has a SCHEDULE attribute. Use the prefetch facility in these cases.

Enabling the asynchronous READ facility

You specify the number of asynchronous READ processes using the pd_max_ard_process operand. If you specify 0 (or do not specify anything) in this operand, the asynchronous READ facility remains inactive. If it is active, you must also specify the prefetch facility (by specifying a value of 1 or greater in the -m option of the pdbuffer operand).