The CICS transaction dump data sets

You can optionally specify DCB parameters for dump data sets if you want to copy the data sets to tape or disk. When CICS opens the dump data set, it issues an MVS DEVTYPE macro. This macro returns the track size for direct access devices, or 32760 for magnetic tape.

The maximum block size used for a transaction dump is the lesser of the values returned from the DEVTYPE macro and 4096. As this usually results in a block size of 4096 (because devices generally have a track size greater than this), CICS writes multiple blocks per track. After writing each block, MVS returns the amount of space remaining on the current track. If the space remaining is 256 bytes or more, then the size of the next block written is the lesser of the values returned by MVS and 4096.

If the space remaining is less than 256 bytes, the next block is written to the next track.