Performance Management Guide

Using SSA

Serial Storage Architecture (SSA) is a high performance, serial interconnect technology used to connect disk devices and host adapters. SSA subsystems are built up of loops of adapters and disks. SSA is a serial link designed especially for low-cost, high-performance connection to disk drives. It is a two-signal connection (transmit and receive) providing full duplex communication. It uses a self-clocking code, which means that the data clock is recovered from the data signal itself rather than being transmitted as a separate signal.

Guidelines for Improving SSA Performance

Examine these guidelines in terms of your situation:

• Limit the number of disks per adapter so that the adapter is not flooded. With high throughputs using large block sizes, five to six disks can flood the adapter.
• Mirror across different adapters.
• The performance will be affected by the location of the logical volume on the disk. A contiguous, unfragmented partition in a logical volume will enhance performance.
• You can turn off mirror write consistency cache for logical volume mirroring, but doing so removes the guarantee of consistency of data in case of a crash. In this case, you would have to recopy all logical volumes to make sure they are consistent. However, the removal does provide a 20 percent plus enhancement in performance.
• For mirrored environments, make sure you are using the parallel scheduling policy.
• If any of the logical volume exists on more than one disk, stagger the partitions over the disks. This is automatically accomplished when the logical volume is created with the inter policy set to Maximum.
• Balance the I/Os across the loop; do not place all the workload on one half of the loop.
• In a multi-initiator environment, place the disks adjacent to the adapter that is using them.

For further information, see Monitoring and Managing IBM SSA Disk Subsystems and A Practical Guide to Serial Storage Architecture for AIX.

Additional information about IBM storage solutions can be found in IBM Storage Solutions for e-business and Introduction to Storage Area Network.