When the checkpoint data set resides on DASD, the change log communicates checkpoint updates to the member that is reading the checkpoint. Because the change log only communicates the changes to the 4K records (not each entire 4K record that has a change recorded on it), checkpoint performance is enhanced. If the change log is small, it will reside only on the first track of the checkpoint data set. This is desirable because it can then be read at the same time as the Lock, Check, and Master Records during the initial (READ1) portion of the checkpoint cycle thereby decreasing the amount of I/O to the checkpoint. If the used portion of the change log is large and spills onto the second track, a second read is required.

You can use the LOGSIZE= parameter on the CKPTDEF initialization statement to control the size of the change log. If you do not assign a value to that parameter, JES2 calculates a value based on the checkpoint mode (DUPLEX versus DUAL; for more information on the modes, see Checkpoint configuration modes) and the number of change log records that fit on track 1 of the checkpoint data set defined by CKPT1= and CKPT2= on the CKPTDEF statement. (See the LOGSIZE= parameter on the CKPTDEF statement for the specific algorithm used to calculate the default change log size.) The LOGSIZE= value can be changed either on a cold start or an all-member warm start.