Introducing SSA and RAID

Introducing SSA and RAID

This chapter describes:

Serial Storage Architecture (SSA)

Serial Storage Architecture (SSA) is an industry-standard interface that provides high-performance fault-tolerant attachment ot I/O storage devices. SSA is an open standard, and SSA specifications have been approved by the SSA Industry Association and are in the process of being approved as an ANSI standard through the ANSI X3T1O.1 subcommittee.

In SSA subsystems, transmissions to several destinations are multiplexed, the effective bandwidth is further increased by spatial reuse of the individual links. Commands are forwarded automatically from device to device along a loop until the target device is reached. Multiple commands can be travelling around the loop simultaneously.
SSA retains the SCSI-2 commands, queuing model, and status and sense bytes.

SSA Loops and Links

In the simplest SSA configuration, SSA devices are connected through two or more SSA links to an SSA adapter that is located in the system unit. The devices, SSA links, and SSA adapter are configured in loops. Each loop provides a data path that starts at one connector of the SSA adapter and passes through a link (SSA cable) to the devices. The loop continues through the devices, then returns through another link to a second connector on the SSA adapter.

The maximum permitted length for an external cable that connects two SSA nodes (for example, disk drives) is 25 meters (82 feet).

Details of the rules for configuring SSA loops are given for each SSA adapter in 'Rules for SSA Loops'.

Loops and Data Paths

All devices that are attached to an SSA adapter card -1- are connected through SSA links -2-. The SSA links are configured in loops. Data and commands to a particular device pass through all other devices on the link between the adapter and the target device.

Data can travel in either direction round a loop. The adapter can, therefore, get access to the devices -3- (disk drives in this example) through two data paths. The system cannot detect which data path is being used.


If a disk drive tails, or is turned off, the loop is broken, and one of the data paths to a particular disk drive is no longer available. The disk drives on the remains of the loop continue to work, but an error is reported to the system.

In the diagram, disk drive number 3 has failed. Disk drives 1 and 2 can communicate with the system only through connector Al of the SSA adapter. Disk drives 4 through 8 can communicate only through connector A2 of the SSA adapter.


If two or more disk drives are turned oft, fail, or are removed from the loop, some disk drives might become isolated from the SSA adapter.

In the diagram, disk drives 3 and 7 have been removed. Disk drives 1 and 2 can communicate with the system only through connector A1 of the SSA adapter. Disk drive number 8 can communicate with the system only through connector A2 of the SSA adapter. Disk drives 4, 5, and 6 are isolated from the SSA adapter.


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