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Chapter 8: Statistics
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Sequential Command
Interval
In determining whether to perform a readahead or not,
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.
Statistic Description