Adaptec 133U2 User Guide - Page 135
Read Caching
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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 Lookahead Cache. Options for these caches can be adjusted separately for each array. s 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. s 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. Lookahead 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 Lookahead Cache. 10-11
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