LaCie 131018 - StorView Snapshot - Mac, Руководство пользователя

Страница: 189 / 214

Statistics
181
15.4 Read-Ahead Statistics
If sequential read commands are sent to the controller, it assumes that the commands which follow may
also be sequential. It can then go and perform a read of the data, before the host requests it. This
improves performance, particularly for smaller reads. The size of the read-ahead is calculated based on
the original command size, so the controller does not read too much data. The controller maintains
statistics for all read-ahead commands performed.
Figure 15–3 Statistics Screen - Read-Ahead Tab
Statistic Description
Sequential Command Interval In determining whether to perform a read-ahead 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 read-ahead cache buffer. In a multi threaded
operating system, commands from one thread may be
interspersed with commands from another thread.
Sequential Command Interval
(continued) 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 read-ahead. 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.