Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 6
There are many terms associated with SCSI. Several versions of the protocol and
several versions of the bus exist. SCSI-1, SCSI-2 and SCSI-3 refer to the protocol to
which a particular device conforms. On the HP 9000 series 800, HP supports devices
that conform to the SCSI-2 protocol only. SCSI-3 specifies an enhanced protocol that
includes, for example, support for 32 LUNs (SCSI Logical Units) rather than the 8
supported in SCSI-2.
SCSI has defined two speeds: Standard SCSI and Fast SCSI. There are two bus
definitions: Narrow and Wide. Both Standard and Fast SCSI have 8-bit data busses
and have a maximum bandwidth of 5- and 10-MB / second, respectively. Fast/Wide
SCSI uses a 16-bit data bus and specifies a maximum bandwidth of 20 MB / second.
Vendors were given the choice of using a single-ended or differential bus when
implementing Standard and Fast SCSI. HP implemented Standard SCSI with a single-
ended bus, which means that there is one wire per signal line referenced to signal
ground. HP implemented Fast SCSI as a differential bus with positive and negative
signal wires referenced together which typically allows a longer cable without severe
signal degradation. Fast SCSI is available on certain HP 9000 700-series workstations
only, using an EISA I/O adapter. Fast/Wide SCSI has always been implemented by HP
with a differential bus, also.
There are two styles of connectors that can be used with SCSI: (1) low-density which
uses an Amphenol-type connector and (2) high-density which is smaller and uses pins.
The high-density connector has become popular since it requires less room on the back
of the computer I/O adapter and the device.
Fibre Channel
Fibre Channel is an emerging networking technology that provides a maximum
bandwidth of 1 Gbit/sec. HP and IBM agreed several years ago to jointly develop a
standard for this technology. The link uses a fiber optic cable to connect the nodes in
the network. It is expected that the maximum distance between nodes in the network
will be 2 kilometers initially, and expanded to 10 kilometers in the future.
A protocol has been defined for Fibre Channel, called the Fibre Channel Protocol for
SCSI for Class 3 services that would allow disk connectivity. Various Fibre Channel
solutions are being developed that will allow disks to be located at distances further
than F/W SCSI. This technology will greatly increase the flexibility in the location of the
disk drives for High Availability and data replication purposes. Some disk vendors are
implementing a native Fibre Channel (FC) interface directly in the disk drive. This
native FC would not make sense for individual disk devices because of the need for
point-to-point connections, but would be useful for large Disk Arrays. HP offers a Fibre