High-Speed Cartridge Tape Positioning

High-speed positioning for cartridge tape is available when opening a tape data set on an IBM® standard-labeled tape for either EXTEND (OUTINX, EXTEND, or DISP=MOD). High-speed positioning is also available when opening to the beginning of such a data set. To invoke high-speed positioning, your program must modify certain fields in the JFCB and use OPEN TYPE=J to open the data set. When you write or read on an IBM 3590 Model A60 at the right hardware level it is not important to use the procedure described here. The magnetic tape subsystem gives these performance benefits automatically. This procedure will not degrade performance.

Tip: On an IBM 3480, IBM 3490, or older models of IBM 3590, this technique offers significantly better performance than the technique for setting the data set sequence number. In addition, systems with DFSMSrmm use this faster technique automatically for all cartridge tapes. For the IBM 3590 Model A60, both techniques give high performance.

Use the following procedure to modify the JFCB:

Issue the RDJFCB macro to have the system move the JFCB into your work area.
Set the JFCPOSID flag in the JFCBFLG3 flag byte to indicate that you are providing a block ID for a high-speed search.
Move the block ID into the JFCRBIDO field of the JFCB. If you are opening to the beginning of a data set, use the block ID of the first header label record of that data set. If you are opening to the end of a data set (for example, to extend it), use the block ID of the tape mark immediately following the last block of user data in that data set.
Issue the OPEN TYPE=J macro to have the system use your modified JFCB.

After the tape is positioned, OPEN processes the trailer labels for the data set being extended.

If you set the JFCPOSID flag off, OPEN positions the volume normally, as though the high-speed positioning feature were not active.

If you set the JFCPOSID flag on, but do not provide a block ID in the JFCRBIDO field, OPEN positions the volume normally and does one of the following:

If you are opening to the beginning of a data set, OPEN inserts the block ID of the first header label record of that data set into the JFCRBIDO field.
If you are opening to the end of the data set, OPEN inserts the block ID of the tape mark immediately following the last block of user data for that data set into the JFCRBIDO field.

OPEN does not update your copy of the JFCB. To retrieve the new value in the system's copy of the JFCB, issue RDJFCB after OPEN.

If the JFCPOSID flag is on during CLOSE processing, (because you set it on before OPEN), CLOSE inserts the block ID for the first header label record of the next data set (which might not exist) into the JFCRBIDC field. Therefore, if you unallocate the cartridge tape device and want to use the current block ID for subsequent processing, save the block ID before you close the data set.

OPEN resets the JFCPOSID flag if any one of the following conditions exists:

Your program issues an OPEN which is not TYPE=J
The requested tape volume is not an IBM standard-labeled volume
The requested unit is not a buffered tape device.

Exceptions:

If you specify dynamic deallocation (with SVC99, FREE=CLOSE on the DD statement, or the FREE option on the CLOSE macro), the block ID for the next data set will not be available to your program.
When using high-speed positioning, specify the data set sequence number normally, either explicitly by LABEL=(seqno,SL) on the DD statement, or by default.

After the system routines have used the JFCRBIDO field for high-speed positioning, they clear JFCRBIDO in the system's copy of the JFCB to prevent misinterpretation during a subsequent OPEN.