HP A3550A - High Availability Disk Arrays Model 20 Storage Enclosure User Manual page 111

Multi-initiator
Two hosts
Two SCSI-2 adapters
(one per host)
One SCSI-2 bus
Two SPs
Multi-bus/
multi-initiator
Two hosts
Four SCSI-2
adapters
(two per host)
Two SCSI-2 buses
Two SPs
C
AUTION
4-4
Adds backup host to two-SP configuration. Applications can continue
after the failure of any disk module. If one host, one SCSI-2 adapter,
or one SP fails, the other host can take over the failed host's disks if
the array and hosts have been appropriately configured with LVM. If
both hosts intend to access a particular disk simultaneously in this
configuration, each host should access the shared disk using the
same primary controller to avoid continuous automatic controller
ownership transfer, which is not supported as a dynamic feature.
Provides the highest availability and best array performance for dual-
host configurations. Applications can continue after the failure of any
disk module. If one host, one SCSI-2 adapter, or one SP fails, the
other host can take over the failed host's disks automatically when
primary and alternate paths have been configured using LVM.