tech-kern archive


Question about the priocscan buffer queue strategy

        hello.  As part of some work I'm doing to determine which disk
strategy to use in a production environment, I've been looking at how the
various strategies work.  The priocscan queuing strategy seems to be quite
good for most work loads, especially work loads where there are a lot of
disperate things going on.  However, I have a question.  It looks like the
priocscan strategy provides for various priority queues, 3 to be exact.  With
that in mind, I have the following questions:

1.  How are i/o requests assigned to the various priority queues?  I see
the calls to select the queue in which to put i/o requests, but I don't
see any code in the kernel that sets the b_prio flag of the queue buffers
to anything at all.  This makes me think all requests end up in the default
low-priority queue.

2.  How are requests sorted within an individual queue?  It looks like
requests within a queue are sorted just as they would be using the disksort
queuing strategy.  Is this correct?

3.  Is there a manual page for this strategy and how to take full advantage
of it?


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