HP Surestore 28/48-Slot with DLT7000 Digital Linear Tape SCSI-2 Drive and SCSI - Page 274

Programmer's Tips Performance Tuning Performance Tuning To enhance the overall tape drive performance, use the drive's buffer memory to optimize read and write operations. The buffer memory is for both the write and read-ahead cache. If one is enabled, the entire buffer memory is available for the enabled operation. If both are enabled, they share the buffer memory without any distinct allocation. Allocate the memory as needed. For example, if the customer use pattern includes many or large writes followed by reads, the writes may utilize all the available buffer space. When the reads start, the tuning parameters force the write cached data to write to tape so that buffer memory is available for read-aheads. After the read operations complete and the write operations restart, the operation system frees the read-ahead cache for needed buffer space. Write Performance Two parameters determine the tape drive's performance during write operations: • The write delay time • The Maximum Burst Size Data stored in the write cache transfers to tape when conditions associated with these two parameters occur. This performance increase is only available when the Immediate Response functionality is enabled. Immediate Response with Buffer Mode Immediate Response means the return of a GOOD status on write operations after all the write data has been transferred from the initiator into the target's data buffer and before the data is transferred to tape. Immediate Response greatly improves write performance by allowing the tape drive to simultaneously perform multiple operations. The drive can transfer write data from one command to the tape while it is evaluating/validating another write request and transferring data for the second write request into the write cache. B- 4 Appendix B