Serial Storage Architecture Management
Miles Purdy
September 2001
Serial Storage Architecture, or SSA, is one of the least understood technologies
I manage. SSA is a fast, high-capacity hard disk storage solution available
for most UNIX-based and PC-based computers, although it may be most predominant
on RS/6000's (a.k.a. pSeries). Many people think that SSA is an IBM proprietary
standard; in fact, it is an open standard. I will explain the technology and
show examples of how to manage SSA on RS/6000's. I will also describe some
benefits of SSA and provide some performance and tuning tips and management
scripts.
The Technology Overview
SSA technology is all about loops and devices talking to each other. (See Figure 1.) Every SSA device is called a node. The nodes in SSA adapters are called
initiators because they issue commands, and the nodes in SSA disks are called
targets because they respond to commands. To create an SSA loop, you start with
an SSA adapter -- a PCI card. Each adapter has two loops (A and B) and two
physical ports per loop (A1, A2, and B1, B2). Copper cables are commonly used
to connect the adapter to an I/O drawer (a.k.a. enclosure). Enclosures contain
a maximum of 16 hard drives. You can start the loop by cabling from port A1
on the SSA adapter to port 1 on the I/O drawer. If the I/O drawer is full of
disks, you can complete the loop by cabling from port 16 on the I/O drawer back
to port A2 on the adapter, thereby forming a complete loop. The automatic bypass
cards will close if they detect a disk on either side, thus closing the loop.
|
