## Understanding read-ahead cache mode

Depending on your controller's level of firmware, there are three settings for read-ahead cache mode:

- Enabled
- Disabled
- Adaptive

You can change the read-ahead setting without destroying data in a logical drive using the ServeRAID Manager in startable-CD mode.

### Enabled read-ahead cache mode

The ServeRAID controller transfers data from disk to its local cache in increments equal to the stripe-unit size. This provides excellent overall performance when workloads are steady and sequential. However, if the workload is random or the system I/O requests are smaller than the stripe-unit size, reading ahead to the end of the stripe might degrade performance.

### Disabled read-ahead cache mode

The ServeRAID controller transfers data from disk to its local cache in increments equal to the system I/O request size, without reading ahead to the end of the stripe. This provides excellent overall performance when the workload is random or the system I/O requests are smaller than the stripe-unit size.

### Adaptive read-ahead cache mode

The ServeRAID controller continually reevaluates whether to transfer data from disk to its local cache in increments equal to the stripe-unit size or in increments equal to the system I/O request size.

The ServeRAID-4 controllers and the ServeRAID-3HB controller come with the adaptive read-ahead mode as a standard feature. The ServeRAID-3H and ServeRAID-3L controllers must use new ServeRAID firmware (3.50, or later) to implement adaptive read-ahead cache mode. You can download and install the new firmware for the ServeRAID-3H and ServeRAID-3L controllers.

If you have the controller and firmware for adaptive read-ahead mode installed, the ServeRAID Manager reports Adaptive on the controller information panel. Otherwise, the information panel states either Enabled or Disabled.

Note: With this new firmware, the non-Adaptive read-ahead modes are no longer available in the ServeRAID Manager. You can override Adaptive read-ahead, but not through the ServeRAID Manager. Instead, use the IPSSEND command-line program. This program comes with the device option and is on the IBM ServeRAID Support CD.