Disk Xfer Rates vs Bus Speed

[Terence Kelleher]

In article <7922 at rpp386.Dallas.TX.US> jfh at rpp386.Dallas.TX.US (The Beach Bum) writes:
>
>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.
>

Since the SCSI bus runs faster than the speed of data to/from media,
"most" SCSI controllers will buffer data, without holding the bus.
Trasfers to a streaming tape will be in bursts, assuming even with
async a SCSI transfer rate of 1.5 MBPS and a buffer to tape speed of
around 390 KBPS.  A tape controller will select when data transfer is
required, but be off the SCSI bus the majority of the time.  A slow
device, say a QIC drive, would take the bus less often and present
less impact to the overall throughput of the bus.  Real impact to
performance comes from very fast devices that can consume a good deal
of bandwidth, but even a good disk will have a 8 ms. typical latency
per command because of physical motion delays when the bus is available.

Of course if the controller is poorly designed, my opinion goes right
down the tubes.  A bad controller can hold the bus doing absolutly
nothing.  I have evaluated a large number of disk and tape devices
with imbedded controllers and these poor controllers are rare.

>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.

Sure, the more resources you put right on the system bus, the better
it runs,  But DMA and system interfaces are expensive.  I admit, in a
Multimax you will get better performance if each SCSI bus has only one
disk, but I can easily put 4 disks and a tape on that same channel and
get good performance.

I have no experience with intelligent ESDI controller.  Are they common?



Terry Kelleher, Encore Computer
UUCP: {bu-cs,decvax,necntc,talcott}!encore!terryk
Internet: terryk%pinocchio at multimax.ARPA