IMS log buffers

When selecting the IMS online log data set (OLDS) block size and number of buffers in order to minimize system log I/O, similar concepts apply from general buffer pool considerations.

The log buffers only need to be plentiful enough to prevent IMS from waiting for a log write in order to get buffer space. The write-ahead data set (WADS) ensures that only full log buffers are written to the OLDS. The log write-ahead function does not truncate log buffers. When defining OLDS block sizes keep in mind that using a multiple of 4K will result in the log buffers being placed above the 2 gigabyte bar in real storage, but are below the 2 gigabyte bar in virtual storage.

Some buffer space is used for dynamic backout. When the backout records exist in the output buffer, they are used directly from the buffers. Otherwise, a buffer is removed from the set of output buffers and used to read the required log block from the OLDS. When backout is complete, the buffer is returned for use as an output buffer. Log buffers are long-term page fixed.

Related reading: See General buffer pool considerations for more information on general buffer pool considerations.