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13. The Extender and Tape Drives
13.1 Using SCSI Fiber Optic Extenders with Remote Tape Drives
The performance of tape drives used with Extenders will vary depending on the
configuration. There are five known variables that significantly change the
performance of the SCSI bus when using tape drives with the Extender.
1. Isolation of the Bus—The first Extender should be connected directly to the
host adapter isolating the bus for the remote devices. There can be multiple
devices daisychained on the remote side.
2. Transfer Block Size—The larger the size, the better the performance will be.
3. Hardware Compression Setting—When Hardware Compression is turned
OFF, performance is increased significantly. We recommend that Software
Compression be turned ON. This provides a method for increasing tape
capacity and increases the performance of the backup procedure.
4. Length of the Fiber—The shorter the fiber, the better the performance will
be. Although, of the known variables, fiber length had the least impact on
overall performance.
5. Number of File Handles—During a backup, the actual number of files will
impact the overall performance. For example, backing up 100 MB of data
contained in 3 files is much faster than backing up 100 MB of data contained
in 100 files.
We recommend the following configuration when using Extenders to remote
tape devices:
• Extenders placed on an isolated SCSI bus.
• Data transfer block size set to 32K or larger.
• Hardware compression turned OFF on the tape drive(s).
• Software compression turned ON. This function is usually included within
backup software programs.
This configuration setting provides the best overall performance. Introducing
the Extenders into this configuration would normally reduce the throughput by 10
to 20 percent.
CHAPTER 13: The Extender and Tape Drives