Adaptec 133U2, User Manual

Page: 135 / 174

10-11
Managing Arrays and Spares
10
Click Save to save the new performance setting. The changes
are not written to disk until you exit the application. If a new
performance setting has been successfully created, it will
appear in the
Optimize Performance for list box.
11
When the Optimize Performance dialog box displays, click OK
to save your changes.
Read Caching
There are two types Read caches: the Demand Cache and the Look-
ahead Cache. Options for these caches can be adjusted separately for
each array.
■
Demand Caching — this defines the maximum Read
command size (stated as a number of data blocks) that will be
cached. Demand Caching can greatly improve I/O
performance if your application typically reads data and then
reads it again soon afterwards. For example, if Demand
Caching is set to 4 blocks and the operating system issues a
Read command for 4 blocks of data, all of the demand data is
cached. however, if the operating system issues a Read
command for 5 (or more) blocks of data, none of the demand
data is cached.
■
Look-ahead Caching Factor — this factor is a multiplier to the
command size that defines the maximum number of data
blocks that will be stored in the Look-ahead Cache in addition
to the amount of data requested by the Read command. Look-
ahead caching can greatly improve I/O performance if your
application typically reads data in sequential blocks. For
example, if the Look-ahead Caching factor is set to 16 and the
operating system issues a Read command, the demand data is
cached if the command is less than or equal to 4 blocks, and the
next (sequential) 16 x 4 blocks of data is stored in the Look-
ahead Cache.