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AIX Commands

SSA for beginners by mrn
Posted: 4 Apr 02 (Edited 24 May 02)

Over the years I've accumulated a large amount of notes on dealing with SSA disk subsystems in an AIX-RS/6000 env. I thought it would be helpful to organise them into an FAQ that served as both a quick reference for the more advanced SSA admin and an entry point to learning for the beginner.

SSA (Serial Storage Archititecture) is IBM's answer to 'new and improved' SCSI. The disks themselves are still SCSI disk drives under the covers. Enhancements come into play with the loop architecture and full-duplex paths. SSA can send and recieve data simultaneously and at full speed, where SCSI sends and receives on the same path. The disks themseleves also are wrapped with SSA specific electronics. The 16-address limitation of SCSI is also overcome with SSA disks. SSA disks are hot-pluggable and do not require special addressing like SCSI disks. Ease of use or introduction to your environment is further augmented by the fact that SSA is fully SCSI-2 compatible. No modification of applications is required.

An SSA loop is the heart of the architecture. It will contain 4 to 48 disks, and at least two adaptor ports. In the most basic loop, which I'll use for illustration purposes, 16 disks are attached to on pair of ports on an SSA adaptor. Each adaptor contains two pairs of ports and is capable of supporting two separate loops - see Fig 1.

In this simple example SSA loop, we have 16 disks numbered 1 to 16 (when you look at an SSA drawer the disk bays in the front are actually labeled 1 to 8 on the front and 9 to 16 on the back). This number is etched into the disk bay cage. The number 1 disk is cabled to port A1 on the adapter, and number 16 is cabled to A2, thus forming a loop.
Because drawers are actually diveded into quadrants, there are implicit connection between disks 4 and 5, disks 8 and 9, and disks 12 and 13.

* A1 -- 1  2  3  4  5  6  7  8       
* A2 \  9 10 11 12 13 14 15 16-|     
*    |-------------------------|     
* B1                                 
* B2                                 
* Fig 1:Simple SSA loop              

This implicit connection is part of the draw electronics. ach initiator(SSA adapter port) can reach any disk in the loop on two paths ( there are two wires in the cable attaching the adapter to the disks). If a disk fails and the loop is broken, the loop will automatically reconfigure itself and reach it destination node(disk) via an alternative path. If you need to remove a disk from the configuration for a period of time, a blank dummy disk is inserted in its place to maintain the loop continuity.

Disk and Disk drawers
There are two styles of disk drawer at present - the original 7133-010 or 020 disk drawer and the 7133-D40. The 010/020 drawers had 2.2Gb 4.5Gb and 9.1Gb disks manufactured for them

I will finish this FAQ in the next week - I promise

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