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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 controller transfers data from the logical drive 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.
More information
v Enabling and disabling read-ahead cache mode (action)
v Changing the stripe-unit size (action)
v Understanding stripe-unit size
Understanding 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 (version 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 properties pane.
Otherwise, the properties pane 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. For more information, refer to the IBM
ServeRAID User’s Reference.
Read-ahead
cache mode settings: Depending on your controller’s level of
firmware, there are three settings for read-ahead cache mode:
v Enabled or Disabled
v Adaptive
You can change the read-ahead setting without destroying data in a logical drive
using the ServeRAID Manager in bootable-CD mode.
Changing the stripe-unit size
Note:
1. This action is supported in bootable-CD mode only.
2. This action is not supported on the ServeRAID-7t controller, ServeRAID
SAS conrollers (8i/8k/8k-l), and the integrated RAID controller.
3. On ServeRAID-7t controller and ServeRAID SAS controllers, use the
configuration wizard to change the stripe-unit size.
Chapter 4. Managing ServeRAID devices 195