Defining a block size for the OLDS

The main factor that determines OLDS block size is the track size of the OLDS devices. The OLDS block size cannot exceed the device track size. Define a block size that maximizes the amount of log data per track (for example, half-track for 3380 or 3390 DASD). Because IMS writes only full OLDS buffers to the OLDS, a large OLDS block size results in more efficient use of DASD space.

You must choose the block size for the OLDS carefully because changing the size of the OLDS after it has been established requires that you stop online work, archive all OLDSs, and scratch and reallocate them to make sure that their block sizes remain identical. When scratching and reallocating, you must delete OLDS entries from the DBRC RECON data set. If you are changing the OLDS block size from being a multiple of 2048 (not running in z/Architecture® mode) to being a multiple of 4096 (running in z/Architecture mode) to ensure that the WADS gets reformatted according to the change in the OLDS block size, you must run the /NRE FORMAT WA or /NRE FORMAT ALL command. After you change the block size of an OLDS, you can restart online work only from an SLDS.

Recommendation: Take a checkpoint soon after the restart so later restarts can use the new OLDS, rather than the SLDS.

The block size of each OLDS must be the same. The OLDS block size must meet all of the following requirements:

The block size must be a multiple of 2048 bytes (2 KB) if IMS is not running in z/Architecture mode. The block size must be a multiple of 4096 (4 KB), if IMS is running in z/Architecture mode.
The block size must be at least 6144.
Recommendation: Use an 8 KB minimum or use the length of the largest message segment.
The block size must not exceed a maximum of 30,720 bytes. This is the largest multiple of 2048 supported by BSAM.
With the support of extended format OLDS, the calculation of how many blocks fit on a track requires that you add an extra 32 bytes of SAM overhead to the IMS block size before making the calculation.

Your OLDS can be more than 65,535 tracks if you use the large sequential data set support. To take advantage of this support, hardware that has more than 65,535 tracks must be used.

During initialization, IMS ensures that the block size specified for the OLDS is large enough to handle the maximum length log record. If the block size specified is too small, IMS discards the OLDS data set looks at the next OLDS. If, at the end of initialization, there are not at least three pairs of usable OLDSs, IMS terminates with an 0073 abend.