HP Surestore 28/48-Slot with DLT7000 Digital Linear Tape SCSI-2 Drive and SCSI - Page 275

Programmer's Tips Performance Tuning The drive flushes the write buffer to tape under the following conditions: • The write hold-off time limit is exceeded (see MODE SELECT command) • Receipt of the following non-write commands: LOAD-UNLOAD REWIND ERASE LOCATE MOVE MEDIUM PREVENT/ALLOW MEDIUM REMOVAL that clears a prevent state • A Write Filemarks command with the immediate bit cleared If Buffered Mode is not selected, the buffer will flush after every write-type command, and the tape drive's transfer rate degrades. Therefore, use MODE SELECT to configure the Buffered Mode. Write operations are written in the order received. The write cache will not reorder writes. Read Performance (Read-ahead) After each read request from the initiator, the tape drive reads additional blocks into the read-ahead cache. When the initiator receives the next read request, the following occurs: 1. Read-ahead cache is checked to see if any blocks are included in the buffer. 2. If all of the blocks are found in the buffer, the data is immediately transferred to the initiator. 3. If there is only a partial hit, only the blocks not in the buffer are read. 4. All blocks transfer to the initiator. Use the Mode Select command to disable read-ahead. Appendix B B- 5