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

Table A: Disk Link Comparisons
Link Link Bandwidth Maximum Link Maximum
(MB/sec) Length Devices per
Link
Peak Sustained
DRAFT -- Revision 2.0
August 22, 1996Page 8
Fibre Channel 1 Gbit / sec 2+ kilometers **** N/A
Fibre Channel- 60 30-40 2 kilometers per 60
SCSI Multiplexer (5000 (2-3 K I/Os Fibre Channel port (15 per F/W
I/Os per per (max 2 ports) SCSI link)
second) second)
* Computer systems do NOT count as devices
** Computer systems DO count as devices
*** Includes cabling internal to the disk drive
**** Distance between nodes in the network or a node and disk, etc.
Although the peak transfer rate for an individual disk mechanism might be higher (e.g.,
10 MB / sec with synchronous transfers on F/W SCSI), most disk mechanisms have
sustained transfer rates in the range from 1.5 to 5 MB / second. To maximize
performance potential, this sustained transfer rate should be used to calculate the
maximum number of active disk drives per link.
The maximum link length will determine the maximum distance between computers in
an HA cluster cabled to the same disks. The F/W SCSI standard limits the maximum
cable length of the bus to 25 meters. This length includes any internal SCSI cabling.
The cable extending from the computer to the first device is usually 2.5 meters.
Therefore, in a two-node cluster, only 20 meters remains for cabling between and
inside the disk trays or towers. There can be as much as 1.75 meters of SCSI cabling
inside disk trays or towers. The system configuration guide shows exact internal cable
lengths.
The maximum number of devices per link is a factor in determining the maximum
amount of disk space that is possible on a given system. This issue will be discussed in
the next section. In the future, Fibre Channel should provide improvements in
bandwidth, distance, and number of devices.
Disk Technologies
Three major disk technologies will be discussed:
Standalone disk drives with LVM mirroring