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

DRAFT -- Revision 2.0
August 22, 1996Page 11
Since there are two internal SCSI busses each with its own connectors, power cords,
power supplies, and fans, LVM mirroring can be accomplished using a single HASS
without having any SPOFs. Each SCSI bus must be connected to a different F/W SCSI
host adapter when mirroring in a single HASS to maintain HA and remove the host
adapter as an SPOF.
As with older JBODs, each storage module in a HASS consumes one SCSI target
address. This limits the total storage capacity possible for a given system according to
the number of system slots and targets per SCSI bus.
The HASS can be ordered with one internal SCSI bus connecting all of the up to 8
modules. This configuration is not supported with LVM mirroring when the mirrors are
in the same HASS since there would be several SPOFs.
The dual SCSI busses can also be used to improve performance when connected to
separate host adapters. Data protection must still be dealt with by mirroring across
HASS chassis.
Although the HASS supports secondary storage modules such as DDS-2 drives and
CD-ROMs and the SCSI busses can be used for either standard SCSI or F/W SCSI,
the secondary storage modules may not be used in a multi-initiator configuration.
When ordering HASS units that will be mounted in cabinets, it is important that the
cabinets be ordered with a second power distribution unit (PDU). This second PDU
provides a second power strip inside the cabinet and a second power cord that can be
plugged into a separate power source. This configuration removes power as an SPOF.
The disk storage modules are hot-pluggable which means that the bus and connectors
are made so that the disk module can be inserted or removed without un-terminating
the SCSI bus. All modules are easily removed from the front of the chassis. The HASS
does not have the problems of previous JBOD configurations that required extra long
F/W SCSI cables, the removal of the chassis from the cabinet, and the removal of the
cover before individual disk mechanisms could be replaced.
However, it is important to note that OS cooperation is still required when removing a
disk module from the HASS since the HASS does NOT provide any data protection or
regeneration of data on a newly replaced disk module. LVM mirroring must regenerate
the data on the storage module after the LVM configuration has been restore with the
vgcfgrestore command.