Overland Storage PowerLoaders AIT-2, User & Installation Manual

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Introduction
t
1-3
SCSI B
US
P
ERFORMANCE
C
ONSIDERATIONS
In a system with many drives, it may be desirable to use multiple SCSI buses for the drives so
the data transfer rate of the drives is not limited by bus bandwidth. Individual drives can be
connected to separate hosts. Using special software, one of the hosts can act as a master server,
processing all robotics commands and permitting several hosts to share a common database.
Configurations
The PowerLoader™ standard SCSI interface is:
•
Fast/Wide Low Voltage Differential (LVD)/Single-Ended (S/E)
The PowerLoader™ uses high-density 68-pin D-series connectors. For more information see
SCSI Interfaces in
Chapter 2 .
SCSI Bus Performance Considerations
Drives
With a standard Fast/Wide SCSI interface, each drive offers a sustained native data transfer
rate of 6 MB/sec. (AIT-2) or 12 MB/sec. (AIT-3). In a two-drive module, the total native rate is
twice these rates. The rates for compressed data are the native rates multiplied by the
compression factor, which depends on file content, but averages approximately 2:1.
Data Transfer Rate
The data transfer rate of the PowerLoader™ depends on the type of AIT drive, the number of
drives, and on how many drives are connected to the SCSI bus. The robotics impose minimal
loading on the bus.
•
AIT-2 Ultra/Wide @ 40 MB/sec. (burst)
•
AIT-2 Sustained @ 6 MB/sec. (native) / 12 MB (compressed)
•
AIT-3 Ultra/Wide @ 80 MB/sec. (burst)
•
AIT-3 Sustained @ 12 MB/sec. (native) / 24 MB (compressed)
Each AIT-2 drive has a maximum sustained rate up to 12 MB/sec. with compressed data. Each
AIT-3 drive has a maximum sustained rate up to 24 MB/sec. with compressed data. As a result,
the transfer rate of a two-drive module occupies the full bandwidth of the bus. The maximum
performance of the system is degraded by sharing a SCSI bus among more than two drives.
Internal Cabling Configuration
Each SCSI bus in the PowerLoader™ System is wired separately. The library robotics and
Drive 1 share one bus; Drive 2 is connected to a separate SCSI bus. SCSI jumper cables are
available to allow the drives and robotics of a module to be daisy-chained to a single SCSI bus.
For more detailed information, see SCSI Interfaces in
Chapter 2 .