<21688 at scuzzy.mbx.sub.org>

[neese at adaptex.UUCP]

>>A few weeks back (late june, I think) there was a thread here on
>>tuning the scsi driver for ISC Unix 2.2. I saved most of those
>>articles since we were planning on installing 1542A's on some machines
>>here.
>>My tests seem to indicate that the default values for BUSON, BUSOFF
>>and DMASPEED are the fastest. 
>
>they are not. however this (partly) depends on the scsi devices you have.
>you *will* experience higher throughput with disks that do read-ahead, caching
>etc. provided you have taken care of various other things (read on).
>it also depends on the other cards you have installed. by increasing a
>cards buson time (and/or decreasing busoff) you are stealing bus cycles from
>the other cards. this can cause problems with floppy controllers etc (you'll
>get 'controller timeout error' etc.). you should ONLY do this if your bus
>has enough total cycles, i.e. is running fast enough to have time for other
>card's DMA. if your bus runs at 6/8MHz the adaptec tends to monopolize the bus
>during heavy disk usage if you set DMASPEED and BUSON too high (and BUSOFF
>too low). especially funny if you use ethernet cards...
>you should configure your motherboards bus speed to 10 or 12 MHz, and, of
>course, you must not set the adaptec's DMA speed to anything higher or equal
>to the motherboards bus speed.

The bus speed has nothing to do with the DMA rate.  It more important to
consider the memory bus rate, which is not driven by the bus clock speed.
As the card is a bus master, it does ignore the bus clock speed and uses
the programmed DMA rate to do all data transfers.  The only signal that can
effect the actual transfer rate is the IOCHRDY signal.

The DMA speed is completely independent of the BUSON time.  If anything, you
would want to set the DMA speed as high as the machine will take it.  I always
recommend 6.7MBytes/sec for the DMA speed as this is much faster than the
SCSI bus can run and will help to minimize any disconnects that may occur
due to the adapter starving the deivce buffer it is connected to.


>also note that no board until now groks 10 MB/s DMA (you can set the old
>aha-1540 to that :-). therefor the max settable speed of the 1540A is 
>8 MB/s.

This is onyl according to the jumpers.  The programmable rate can be as high
as 10MBytes/sec.

>>I've installed the Sync option jumper on the controller. 
>
>that jumper doesn't matter - it's setting is overridden by ISC's scsi
>driver.

Software cannot override this jumper.  This jumper simply indicates who will
attempt to negotiate for synchronous protocol.  If the jumper is installed the
AHA-154x board will negotiate for it, if the jumper is removed the adapter
expects the device to negotiate for it.  I would allow a device to do the
negotiation as some devices will do the negotiation on each data transfer.
If the 154x does the negotiation, it will only be done once.

>>With BUSON 5, BUSOFF 9 and DMASPEED 0; the system seems to be
>>performing just fine. 
>
>sure, these are the 'works 100%' parameters. not the 'works 100% and
>performs 100%' paramters.

The system will perform with these paramters, but not they are not the
optimum values.  It actually takes some experimentation to find what is
best for any given environment.  The variables that come into play are
the device(s) connected, the size of any given data request, and the
actual speed data can be transfered across the AT bus.

>>STUFF DELETED<<
>>During these tests I've left the DMA speed selection header on the
>>board open. This selects the default of 5 Mhz. I'm going to try
>>other selections on the jumpers. Is there a reason why this is not a
>>good idea?
>
>these jumper settings are also overridden by the software.

Correct.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  uunet!swbatl!
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