Sequential buffering introduction

OSAM sequential buffering performs a sequential read of ten consecutive blocks with a single I/O operation, while the normal OSAM buffering method performs a random read of only one block with each I/O operation.

Without SB, IMS must issue a random read each time your program processes a block that is not already in the OSAM buffer pool. For programs that process your databases sequentially, random reads can be time-consuming because the DASD must rotate one revolution or more between each read.

SB reduces the time needed for I/O read operations in three ways:

• By reading 10 consecutive blocks with a single I/O operation, sequential reads reduce the number of I/O operations necessary to sequentially process a database data set.

  When a sequential read is issued, the block containing the segment your program requested plus nine adjacent blocks are read from the database into an SB buffer pool in virtual storage. When your program processes segments in any of the other nine blocks, no I/O operations are required because the blocks are already in the SB buffer pool.

  Example: If your program sequentially processes an OSAM data set containing 100,000 consecutive blocks, 100,000 I/O operations are required using the normal OSAM buffering method. SB can take as few as 10,000 I/O operations to process the same data set.
• By monitoring the database I/O reference pattern and deciding if it is more efficient to satisfy a particular I/O request with a sequential read or a random read. This decision is made for each I/O request processed by SB.
• By overlapping sequential read I/O operations with CPC processing and other I/O operations of the same application. When overlapped sequential reads are used, SB anticipates future requests for blocks and reads those blocks into SB buffers before they are actually needed by your application. (Overlapped I/O is supported only for batch and BMP regions.)