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From: karl@robot.in-berlin.de (Karl-P. Huestegge)
Newsgroups: comp.periphs.scsi,comp.unix.sysv386
Subject: Re: parameters on CDC Wren IV, Adaptec 1542b
Message-ID: <1990Nov6.194432.6998@robot.in-berlin.de>
Date: 6 Nov 90 19:44:32 GMT
References: <1990Nov3.185524.468@unixland.uucp> <1990Nov04.004934.232@nstar.uucp> <1990Nov4.143952.5251@unixland.uucp>
Organization: K.-P. Huestegge, Sytemanalyse und Beratung, Berlin
Lines: 53

bill@unixland.uucp (Bill Heiser) writes:

>larry@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@robot.in-berlin.de
Berlin Friedenau                           ..unido!fub!geminix!robot!karl