Specifications

4 Optimizing IBM Netfinity Servers for SAP R/3 and Windows NT
Figure 1. Bidirectional SSA Loop
SSA connectivity is extremely adaptable and it requires only thin copper cables to
connect devices together. As SSA is a serial communication protocol it only
needs two twisted-pair copper cables for a full-duplex data transfer between two
devices. Because you can use thin copper cables, the quality and the protection
of the cables is much better than with SCSI. You get an improved signal quality
that allows higher clock rates on the cable and distances of up to 25 meters
between adapters, drives or subsystems with copper cables. With the SSA
Fiber-Optic Extender feature installed on the SSA subsystems 7133 you can even
reach up to 2.4 km (7874 feet) by using fiber optic cables. This flexibility allows
you to position storage subsystems in secure locations and mirror subsystem
over long distances.
SSA has link error recovery procedures and an automatic path selection for
alternative paths. Through a loop configuration, SSA prevents a cable failure from
exerting influence on access to data, making it a good choice when
high-availability implementations are needed. When data is sent from the
adapter, it may transfer over the cable in either direction. If SSA detects
interruptions in the loop, it can automatically reconfigure to preserve the
connection. When the faulty cable is replaced the loop will automatically
reconnect. This ability is also useful to extend a running system. You can open
one connection and add an additional subsystem and close the loop again. The
server need not be brought offline, as all SSA cables and disk drives are hot
swappable.
In addition, SSA offers you some other unique capabilities. The IBM SSA RAID
PCI Adapter and IBM SSA RAID Cluster Adapter have four ports to build two
loops. You can connect up to 96 SSA disk drives per adapter; this means 48 disk
drives per loop.
The IBM SSA RAID PCI Adapter supports the implementation of RAID-0, RAID-1
and RAID-5. The drives can be configured in as many as 32 arrays, each with 3 to
16 disk drives with RAID-0 and RAID-5, and in pairs with RAID-1. The adapter
can also support non-RAID drives.
The IBM SSA RAID Cluster Adapter is developed particularly for failover cluster
support with Windows NT and the Microsoft Cluster Server (MSCS). You can
create up to 22 RAID-1 arrays in disk pairs and non-RAID drive configurations are
also supported.