#SCSI Problem.
Yes, I suspect that his explainations are reasonable. Indeed, usually
drive errors are reported as "mode sense" errors or "page xxxx" errors
(where "xxxx" means I don't remember the correct term just now).
It seems like the scsi bus driver should automagically NOT request
"reselect mode" for non-disk type devices. In any event, your overall
throughput may be higher with reselection off for all devices than with it
on. This is very dependent upon how the system is being used as well as
specific drive performance considerations.
Basically, reselection allows multiple scsi bus operations to be occuring
at the same time. For most single user systems this ability will not make
any significant difference. On the Amiga it further may not matter since
the OS will queue up multiple unrelated actions and interleave them
between blocks of complete actions.
For example, if you were copying a 10Meg file from one drive to another:
For the situation where the entire file will be copied in one operation,
without reselection the entire scsi bus would be unavailable until the
command completed.
However, in the first place, the Amy OS will not copy a 10 Meg file in one
operation. Thus, following each individual read operation or write
operation there is an opportunity for Ami to "sneak in" an unrelated
operation (which in itself will cause the pending copy to wait until this
interrupting operation is complete).
With reselection however, the bus is available as soon as a command has
been acknowledged. Thus, even for Amy, if a read (for our example) were
issued to obtain say 500K of data, the drive would acknowledge the receipt
of the command. At that point Ami could then go on to a different
operation with a different drive since the first drive will opt for
attention only after it is ready to transfer the requested data.
In a single drive system (at least with SCSI-I), reselection ALWAYS slows
the system down since only one operation can be pending for any one drive
at any one time (on the bus). I say "at least with SCSI-I" because while
I don't think that the situation has changed for SCSI-II I am not certain
about that.
-bill