Subject: Re: My $0.02
To: Tobias Abt <>
From: Theo de Raadt <>
List: amiga-dev
Date: 08/04/1994 02:34:59
> > > You should be advised: slow devices at high scsi id's fast ones
> > > at low scsi id's.
> > 
> > Can you please explain this? I always thought all devices on the SCSI-bus have
> > equal rights?
> No, the unit ID is also the priority. This is the reason why you usually set
> the host adaptor to seven. :-)


When vying for access to the bus (during a particular SCSI bus phase..),
each SCSI device turns on the SCSI bus data bit that corresponds to
it's SCSI id. The highest bit wins.

Tape drives generally have small buffers. They also normally have to
get the tape up to speed before they can read or write. This is why
some tape drives do that "whir bzz whir" sound -- they are zipping
back and forth. Each time you "starve" a tape drive, it will slow
down, stop, back up, get back up to speed, and then write the next set
of blocks. What a waste of time eh? (This is another reason why HP's
high end DAT drives are so awesome -- 1M of buffer).

Say you are doing a backup; if your tape drive is at a higher priority
(set the SCSI id to 4 or 5) it will win relative to the disk drive
that you are reading data off. If it wins, it gets fed data. If it
gets fed data, it "streams".