parameters on CDC Wren IV, Adaptec 1542b

[Karl-P. Huestegge]

bill at unixland.uucp (Bill Heiser) writes:

>larry at nstar.uucp (Larry Snyder) writes:
>>what you need to do is to change one item at a time - then
>>bench the system, change another - then bench, etc..   

>Thanks for the feedback Larry.  What utility are you using to do 
>your benchmarking?

>>I have cache enabled (made a 10% difference) and changed the
>>buffer ratios from 4 to 16 (made a + difference on large disk IO,
>>and - on small; but since most of mine is large - I changed it)..

>I'm a little uncertain about the "buffer ratios" -- I didn't see any
>explanation of it in the lenghy aha1542 manual -- do you have anything
>that describes them?  My "largest" disk IO activity is news processing
>(Cnews) and compiling software -- does that qualify as "large", and 
>would you expect it to benefit from the increase in buffer ratios?

The Host Adapter has nothing to do with it - it's a SCSI-command.

The Buffer-Full Ratio and Buffer-Empty Ratio are part of the Mode Select
Command called the 'Dissconnect/Reconnect Control Page'.

The Buffer-Full (Buffer-Empty) Ratio indicates on READ (WRITE) operations 
how full (empty) the buffer should be prior to attempting a reconnection.

The Quantum Prodrive Specs say:

Buffer-Full Ratio: During READ Operations, the drive disconnects when
    the buffer is empty. The Value of the buffer-full ratio represents
    the percentage of the buffer that must be filled, unless the buffer 
    contains all the required data, prior to reconnection of the drive
    - that is, 255 (FFh) represents 100% full; 128 (80h), 50% full.
    When the value is zero, the drive determines when it will initiate 
    reselection. The default value is 80h, indicating that the drive
    will attempt a reconnection whenever the buffer contains at least
    four kilobytes of data read from the disk.

Buffer-Empty Ratio: When the drive is disconnected during WRITE 
    operations, the value of the buffer-empty ratio represents the 
    percentage of the buffer that must be empty to enable
    fetching data from the initiator, unless the buffer can hold
    all the required data - that is, 255 (FFh) represents 100% empty,
    128 (80h) 50% empty. When the value is zero, the drive determines,
    when it will initiate reselection. The default value is 80h, 
    indicating that the drive will attempt a reconnection whenever
    the buffer contains at least four kilobytes of data transferred
    from the initiator.
    
-- 
Karl-Peter Huestegge                       karl at robot.in-berlin.de
Berlin Friedenau                           ..unido!fub!geminix!robot!karl