Using the Improved Data Recording Capability

Improved data recording capability (IDRC) is a data compaction feature of the 3480 subsystem, and part of the base model of the 3490 and 3490E cartridge controller. Data compaction in the cartridge control unit eliminates the CPU overhead from software compression. IDRC-compacted data is created using a technique called auto-blocking, which ensures optimal physical blocking for cartridge media. The block size is unaffected by this technique. A typical compaction ratio is 3:1; however, inefficiently blocked tape data sets can achieve more dramatic compaction.

IDRC helps to reduce the number of cartridges required to store data, and reduces the elapsed time for batch jobs depending on cartridge I/O. This makes IDRC effective for both single volume and multivolume data sets. It optimizes the data exchange between the controller and the device, increasing the number of devices that can be used concurrently.

To implement IDRC with SMS data classes, perform the following steps:

Define data classes for your offsite tape data and very large backup, active, and temporary tape data sets. Set COMPACTION to Y for these data classes. Data classes for these tape data set categories are TAPOSITE, TAPBKUP, TAPACTV, and TAPTEMP.
Allow your data class ACS routine to assign these classes during allocation. This method writes the data in IDRC-compacted format. The same result occurs if you specify DCB=TRTCH=COMP on the DD statement.
Remember that the data class ACS routine is driven for both system-managed and non-system-managed data sets.

Using data class lets the system determine an optimal block size for the tape data set if you do not specify one. z/OS DFSMS Using Data Sets describes how the system determines block size for tape. Using system-determined block size improves tape channel usage and buffer management.
Define a data class, NONTMM, with COMPACT=N, so tape data sets are directed to tape in a non-compacted form. Use the NONTMM data class on your DD statements to tell your data class ACS routine that the data sets should not be compacted or redirected to the DASD buffer. These might be data sets shipped offsite to facilities without IDRC-capable drives or those used by applications that call the READ BACKWARDS command. This command is simulated for IDRC data sets; compacting them severely degrades performance. Few data set types rely on the READ BACKWARDS command. For example, the IMS™ log is read backwards if it is processed during recovery. Do not compact DFSORT work files.