Scsi Bus Performance Considerations; Data Transfer Rate; Drives; Internal Cabling Configuration - Overland Data DLT LibraryXpress Series Service Manual

SCSI Bus Performance Considerations

Data Transfer Rate

The data transfer rate of the DLT LibraryXpress is dependent on the type of drives and on the
SCSI bus configuration. The bandwidth of the SCSI bus is limited. As a result, the transfer
rate of exceptionally fast drives such as the DLT 7000 series can be limited by sharing a
SCSI bus with another drive.

Drives

With a standard SCSI interface, modules equipped with 2000XT series drives can provide a
sustained native transfer rate of 1.25 MB/second. With both drives operating, the 2000XT's
rate is 2.5 MB/second. For the 4000 series, the maximum sustained native rate is 1.5
MB/second. With both drives operating, the 4000's native rate is 3.0 MB/second. In modules
equipped with 7000 series drives, the native data transfer rate is 5.0 MB/second, 10 MB/sec
for two drives. The rates for compressed data are the native rates multiplied by the
compression factor, which depends on file content, but averages approximately 2:1.

Internal Cabling Configuration

The LibraryXpress System is wired in an independent-bus SCSI configuration. That is, the
robotics and each of the drives has its own SCSI bus. SCSI jumpers are supplied for those
applications requiring that some combination of drives and robotics be daisy-chained to a
single SCSI bus. These configurations are described in more detail in the section on SCSI
Interfaces in Chapter 2 - Installation.

Bus Length Limitations

The single-ended SCSI-2 bus is limited to 6 meters overall length, and the single-ended Fast
SCSI-2 bus is limited to 3 meters overall length. In contrast, differential SCSI-2 and Fast
SCSI-2 buses may be up to 25 meters. The internal wiring of the modules in a single-ended
LibraryXpress system may consume a significant fraction of the allowed length, leaving very
little extra for interconnection between modules and for attachment to the host. Overland Data
strongly urges use of the differential configuration for best performance.

Physical Configuration

All modules in a LibraryXpress System must be mounted in a rack. When a system is ordered,
all of the modules are supplied in rack mount configuration. None of the modules may be
operated alone, except for the LibraryXpress Base Module, which may be converted to
operate as a free-standing 10-slot library. For users who wish to remove a Base Module from
a system and use it alone, a conversion kit is available to convert a rack mount module to
desktop configuration. Another kit is available for conversion in the opposite direction. See
Appendix B - Accessories/Spares/FRUs for the part numbers.
4 Chapter 1 - Introduction