Disk Xfer Rates vs Bus Speed

[The Beach Bum]

In article <3877 at encore.UUCP> terryk at pinocchio.UUCP (Terence Kelleher) writes:
>ESDI is an easy winner, if you compare 1 disk to 1 disk.  If you are
>using more disks, SCSI provides the nice features of overlapped
>operations.  2 disks, with seperate [ sic ] controllers, on the SCSI bus will
>provide very near twice the throughput of 1 drive.  This is not the
>case with ESDI.  Which is better really depends on wheter you care
>more about overall throughput or single access times.

Adding a slow I/O device to the SCSI bus is a sure way to slow the entire
bus down.  The SCSI bus supports at most one data transfer occuring at any
single time.  If a large streaming tape [ as one good example ] transfer
is presently occuring, all disk transfers are suspended.  Also, during the
data phase for the presumed tape request, no other SCSI commands may be
issued.

You would have to provide separate SCSI buses for each device to insure
that no one device hogged the bus, or put slow devices on separate buses
and leave the disks on one single bus.

This is all moot anyhow since multiple ESDI disk controllers can be placed
on the same system bus [ ghod willing ], and this would permit multiple
simultaneous seeks and data transfers [ assuming DMA ].  Further, intelligent
ESDI controllers exist which support overlapped seeks, which was Terrence's
original argument for the SCSI bus.
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