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

DRAFT -- Revision 2.0
August 22, 1996Page 20
multiple internal SCSI busses
deskside and rackmount models
redundant and hot-swappable:
power supply
fan module
dual storage processors optional on Models 10 & 20
dual storage processors (optional) on Models 10 & 20 for performance
redundant storage processor(s) in case of failure
Cost, capacity and the features listed above make the HADA the practical choice in
many situations where several hundred Gigabytes of storage is required. HADAs can
be configured in RAID 0, 1, 0/1 and 5. Although the hardware supports RAID level 3,
that level is not supported on an HP-UX system due to the generally poor performance
in comparison to the other RAID levels. Note that on-line disk replacement is supported
only in RAID levels 1, 0/1, and 5.
Disk mechanisms are grouped together and assigned to a SCSI LUN, which is another
level of addressing beyond the SCSI target address. In HP-UX, there is a limit of 8
LUNs (0 through 7) per SCSI device. Adding new disk mechanisms does not require a
reload of the data since new disks may be assigned to a separate LUN. Each LUN can
be configured for any supported RAID mode.
Internally, the Model 10 HADA has two standard speed SCSI busses with up to 5 disk
mechanisms on each bus. Data is striped across disks on the same SCSI bus.
Internally, the Model 20 HADA has five standard speed SCSI busses with up to 4 disk
mechanisms on each bus. Data is striped across disks on different SCSI busses.
Both the Model 10 and the Model 20 arrays come with one storage processor and a
second one can be added. When the second optional storage processor is added,
disks can be assigned in groups to either storage processor.
Each active or redundant storage processor requires a SCSI target address. So, a
HADA requires one or two SCSI target address depending on whether a second
storage processor has been purchased. The second storage processor can be used
both to distribute the I/O load for increased performance and for redundancy. Initially,
each storage processor is assigned certain disk mechanisms in specific RAID modes.
Upon failure of one of the storage processors, the surviving storage processor can take
over all of the disk mechanisms.
HP-UX version 10.0 introduced a new capability to LVM called PV Links. With PV
Links , LVM detects that there are multiple paths to a disk or array. The first device file
configured with vgcreate or vgextend becomes the primary path. The device file
corresponding to the alternate path then becomes the redundant path. In case of