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

DRAFT -- Revision 2.0
August 22, 1996Page 28
Multiple system connections are not made with multi-initiator busses. Rather, separate
connections from each system is made to the Symmetrix using different SCSI ports on
the Symmetrix.
The Symmetrix can be configured with up to 128, 4 GB or 9 GB disk mechanisms
assigned to up to 256 LUNs that can be configured on RAID 1 and RAID S groups.
Internally, there are 32 SCSI busses (4 per disk director) with up to 4 disk mechanisms
on each bus. There can be up to 8 Channel Directors that can be a combination of
Open Storage Directors that provide 4 F/W SCSI ports for Unix hosts and Remote Link
Directors for SRDF configurations.
The Symmetrix must be connected to HP 9000 host systems in pairs. That is, at least
two F/W SCSI links must be connected to a host system for redundancy purposes.
Both links can be used to access the data. In the event that one link fails, the other link
can be used to access all of the data in conjunction with the PVLinks feature of the
operating system, described in a previous section. Symmetrix does not have the
performance impact that the HADA has when switching between controllers for the
same LUN. Note that PVLinks does not currently allow concurrent access to the same
LUN via both paths. The second path is for redundancy, in case of failure of the host
adapter, cable or Symmetrix Open Storage Director. PVLinks does permit concurrent
access to different LUNs via both paths. To take advantage of this capability, multiple
LUNs must be defined in the Symmetrix.
Only one Symmetrix is allowed per F/W SCSI host adapter. No daisy chaining is
allowed. Figure 7 shows the only supported configuration for Symmetrix when
configured in an HA cluster. This configuration was established for both performance
and availability reasons. There are no SPOFs in this configuration since each Open
Storage Director is attached to a different SCSI bus and host adapter. The combination
of PV Links and dual Open Storage Directors provides very similar availability as
JBODs in association with the HA software MC/ServiceGuard and MC/LockManager.