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

DRAFT -- Revision 2.0
August 22, 1996Page 50
Power source redundancy
To ensure the highest level of availability, data should be protected by multiple
power sources. This is possible today only with standalone mirrored disks. The
loss of one power source or circuit will not prevent access to the data. Of
course, this only makes sense if used together with MC/ServiceGuard where the
systems that can run the application are powered separately, also.
Total distance among computer systems connected to the disks
The more systems connected to the same set of disks, the longer the total cable
length required. Although HP-FL allows the longest cable length (up to 500
meters from each system to the string of disks), its performance is much lower
than newer technologies such as F/W SCSI.
The total cable length for F/W SCSI is limited to 25 meters, including all cabling
inside towers and trays. Since each system in the cluster consumes 2.5 meters
of cabling and each "V" cable consumes an extra 2 meters, a four-node cluster
will consume 16 meters of cable not including any internal cabling.
DISK SELECTION MATRIX
Table M summarizes the decision criteria for selection of the appropriate disk
technology. Each of these criteria involves a business decision that should be made.
These criteria should also be ranked when more than one is required.
Table M: Disk Selection Matrix by Technology
Technology Criterion If Required, then If Not a
choose from Requirement,
this list of then choose
options from this list
on-line replacement of failed disk RAID 0/1 any
mechanism RAID 3
RAID 5
global hot spare disk mechanism for RAID 0/1 any
automatic reassignment in case of failed disk RAID 5