SCSI Solutions White Paper - HP-UX

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Executive Summary
This paper provides an overview of parallel SCSI (pSCSI) technology and solutions on HP-UX.
A basic understanding of mass storage technologies is assumed. Topics discussed include:
• pSCSI history and futures
• HP pSCSI products and solutions
• Configuration rules and guidelines
• Online repairability and High Availability (HA)
• Enclosure management
• Performance and feature comparisons
1 SCSI Overview
1.1 History
SCSI (Small Computer System Interface) is an ANSI standard for interconnecting computers to
peripheral devices such as disks, tapes, CD ROMs, and autochangers. In its original form,
known as SCSI-1, it was a narrow (8-bit) parallel bus with a 5 MB/second maximum transfer
rate which supported only single-ended transmission with passive termination.
SCSI-2 provided a higher-speed option known as Fast SCSI, a wide (16-bit) bus option known
as Wide SCSI (which allowed for doubling the number of devices per bus that could be
attached, and a doubling of the raw performance, which together with the Fast SCSI option
allowed for quadrupling of the raw performance), improved connectors and cabling, more
reliable termination known as Active Termination, differential signaling (which later became
known as High Voltage Differential – HVD) to allow for longer cable lengths, command queuing
for increased performance (pipelining requests to a device) and support for additional types of
devices such as removable media, CD-ROMs, and scanners.
SCSI-3, the latest version of the standard, expanded the SCSI horizons beyond the parallel bus
to serial technologies, such as Fibre Channel and more recently iSCSI and SAS, separating the
underlying transport technologies from the SCSI command protocol. On the parallel bus,
SCSI-3 brought significant performance and feature improvements, initially defining Ultra SCSI
at double the Fast SCSI transfer rate, and then successively Ultra2, Ultra3 (also known as
Ultra160), and Ultra320 SCSI, each doubling the raw speed of the bus.
1.2 Key Features
The basic features evolved with the data transfer rate options, as shown in the table below. In
this context MB/s is 1,000,000 bytes per second, and MT/s is 1,000,000 Transfers per
second. On a narrow (8-bit) bus 1 MT/s = 1 MB/s; on a wide (16-bit) bus 1 MT/s = 2 MB/s.
For all speeds other than Ultra160 and Ultra320, 1 MT/s = 1 Mhz clock speed on the bus.
Ultra160 and Ultra320, however, use double-transition clocking (sampling data on both the
rising and falling edges) to do two transfers per clock cycle.
There are three types of electrical signaling used in SCSI buses: