Chapter 8: Statistics
Statistic | Description |
|
|
Sequential Command | In determining whether to perform a readahead or not, |
Interval | the controller will search back in the command queue |
| whenever it receives a new read command that is not |
| satisfied by an existing readahead cache buffer. In a |
| multi threaded operating system, commands from one |
| thread can be interspersed with commands from |
| another thread. This requires that the controller not just |
| check the immediately previous command. The |
| controller will search back for a number of commands, |
| to see if the new command is exactly sequential to any |
| one of these previous commands. If it is, then the |
| controller determines that the data access pattern is |
| sequential, and so performs a readahead. These |
| statistics record the average number of commands the |
| controller must search back for when it finds a |
| sequential command match, the maximum number, and |
| also the percentage for each one of these values. These |
| give an indication of the multi threaded nature of the |
| host. |
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162 | www.gateway.com |