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

DRAFT -- Revision 2.0
August 22, 1996Page 7
Channel-SCSI Multiplexer, enabling existing F/W SCSI disks to be located up to 2
kilometers from the host computers.
The Fibre Channel-SCSI Multiplexer is henceforth called the FC/SCSI Mux for brevity.
The FC/SCSI Mux can have two Fibre Channel (FC) ports and that capability is very
important for High Availability, since it provides a redundant link to the disks if the
primary link goes down. Alternatively, the second FC port can be used to connect to a
different host in a High Availability Cluster. Because of the increased distance, the
FC/SCSI Mux can be used to implement campus-based Disaster Recovery (DR)
solutions that would simplify the recovery from failures affecting an entire data center,
for example.
The FC/SCSI Mux has four slots, each of which can contain a card that connects to a
separate F/W SCSI bus with up to 15 disk devices on it. So, a single Fibre Channel link
on a host can provide connectivity to up to 60 disk devices using a point-to-point
topology. Throughput on the FC/SCSI Mux peaks at 60 MB/sec (or 5000 I/Os per
second), providing the throughput of three host-based F/W SCSI cards and the
connectivity of four host-based F/W SCSI cards but using only a single slot in the host.
Thus, hosts with fewer slots can now connect to greater capacities of disk storage.
However, host I/O bus bandwidth must be considered when determining the practical
amount of disk storage. This issue is discussed further in the section on Performance
Comparisons.
This new technology provides vastly increased flexibility in location of disk devices,
connectivity to multiple hosts, and high bandwidth using a single host adapter slot.
Disk Link Comparisons
The choice of I/O link depends on the particular disk and requirements of distance,
performance and capacity. These factors are summarized in the Table A.
Table A: Disk Link Comparisons
Link Link Bandwidth Maximum Link Maximum
(MB/sec) Length Devices per
Link
Peak Sustained
HP-FL 5 4.7 500 meters 8 *
Standard SCSI 5 2.5 6 meters *** 8**
F/W SCSI (HP-PB) 20 7 - 10 25 meters *** 16**
F/W SCSI (HSC) 20 12 - 15 25 meters *** 16**