HP storageworks HSG80 Reference Manual page 165

Read caching improves performance in almost all situations. Therefore, it is
recommended that you leave its default setting, READ_CACHE, enabled.
However, under certain conditions, such as when performing a backup, read
caching may not be necessary because the data will probably not be re-read. In
such instances, it may be beneficial to disable the read cache function and remove
the processing overhead associated with caching data.
READAHEAD_CACHE (default)
NOREADAHEAD_CACHE
Enables the controller to keep track of read I/Os. If the controller detects
sequential read I/Os from the host, it will then try to keep ahead of the host by
reading the next sequential blocks of data (those the host has not yet requested)
and put the data in cache. This process is sometimes referred to as prefetch. The
controller can detect multiple sequential I/O requests across multiple units.
Read-ahead caching improves host application performance since the data is read
from the controller cache instead of disk. Read-ahead caching is the default for
units.
If you have a unit that is not expected to get sequential I/O requests, select
NOREADAHEAD_CACHE for the unit.
RUN (default)
NORUN
Controls the unit's availability to the host.
• Specify RUN to make a unit available to the host.
• Specify NORUN to make a unit unavailable to the host and to cause any data
in cache to be flushed to one or more drives. NORUN spins down all the disks
used in the unit. The drives making up the unit spin down after the data has
been completely flushed.
NOTE: Regardless of storageset type, the NORUN switch cannot be specified for units that
are partitioned.
NOTE: Specifying NORUN and then RUN to a unit also causes the subsystem
configuration to be updated to the selected disk unit.
WRITE_LOG
NOWRITE_LOG (default)
Marks the defined unit as a Host Based Volume Shadowing shadow set member.
NOTE: This switch is only supported with ACS Version 8.7R or Version 8.7W.
NOTE: The specified unit is only marked and not enabled.
HSG80 Array Controller V8.7 Command Line Interface Reference Guide
SET unit-number
2–149