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

DRAFT -- Revision 2.0
August 22, 1996Page 15
(1) One controller may be designated as a master controller with the other
controllers acting as slaves. This method was implemented in the HP-FL array
models C2258HA and C2259HA and the Fast/Wide SCSI array models
C2439JA/JZ and C2440JA/JZ.
(2) The second method incorporates a higher level storage processor that
controls the complete functions of the RAID array. The High Availability Disk
Array models A3231A (Model 10) and A3232A (Model 20) have either one or
two storage processors besides the individual disk controllers. The Disk Array
with AutoRAID models A3515A and A3516A have two controllers. These
storage processors or controllers control caching, RAID level, sparing and data
recovery.
Each disk array appears to the operating system as one large disk or several large
disks using SCSI LUNs (Logical Unit Numbers). This LUN has nothing to do with the
HP-UX 9.X LU (Logical Unit) that was an index into an OS table that was used to keep
track of devices connected to the system. In HP-UX 10.0, card instance numbers are
used in place of device LU numbers.
Each SCSI LUN addresses one or more physical disk drive(s) inside the array. It may
be fixed by the array firmware or be configurable by the system administrator. LUNs
will be discussed further in the sections on disk arrays.
There were five RAID levels defined by the Patterson, et al. paper. They were:
RAID level 1 mirrored disks
RAID level 2 multiple check disks using Hamming Code
RAID level 3 single check disk using Parity, byte interleaved (also called
byte striped)
RAID level 4 single check disk using Parity, sector interleaved
RAID level 5 no single check disk, data and parity spread across all disks,
sector interleaved (also called block striped)
Since the paper was written, several new RAID levels have been defined:
RAID level 0 no check disk, no data protection, sector interleaved (also
called block striped) across a group of disks