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

DRAFT -- Revision 2.0
August 22, 1996Page 17
chained together, each supports the connection to a computer system, thus supporting
a total distance of 1 km end-to-end. The physical link peaks at 5 MB / sec.
On-line replacement of failed disk mechanisms is available in RAID 3 mode only . Since
RAID level 3 provides relatively poor performance on HP-UX systems, these arrays are
most often used in mirrored configurations using LVM mirroring, with the array setup in
Independent Mode (a special case of RAID 0). Unfortunately, on-line replacement of
failed mechanisms is not supported in this configuration. So, although on-line
replacement of failed disks with RAID level 3 improves HA, performance is so
negatively affected, that HP recommends the use of these arrays in Independent Mode
(RAID 0) together with LVM mirroring. This configuration would increase total capacity
over JBODs since the array consumes only one target address but provides 10 GB of
disk space.
It is suggested that the only reason for choosing FLDAs is compatibility with or reuse of
existing disk storage despite the lower performance of the HP-FL link and the lower
RPM disk drives. Due to the long cable length of HP-FL, It is possible to connect these
disks to systems located in different buildings, even up to 500 meters away. However,
Fibre Channel disk storage solutions provide an even greater distance.
Figure 3 shows how HP-FL can be configured for a High Availability environment. HP-
FL allows up to 8 FLDAs on the link. Performance considerations may limit the number
of arrays to less than 8. This configuration can be used with HA software as discussed
in the section on JBODs.