Disk Xfer Rates vs Bus Speed

[markb at denali]

In article <3900 at encore.UUCP>, terryk at pinocchio.Encore.COM (Terence Kelleher) writes:
> In article <7922 at rpp386.Dallas.TX.US> jfh at rpp386.Dallas.TX.US (The Beach Bum) writes:
> 
> 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.
> 
If you are in fact running async SCSI, your xfer rate from the media can
exceed the xfer rate of the SCSI bus.  You should also keep in mind that
connects and disconnects suck your xfer rate down.  Again let's consider
xfer rate-burst is substantially different from throughput.  Again because
of inherent comparative slowness of SCSI to other buses, throughput will
be low if small buffers are on your deviec level controllers.
 
> I have no experience with intelligent ESDI controller.  Are they common?
> 
Pretty much on VME, somewhat less sophisticated on multibus, and probably
rare on PC bus, and the like.

					markb

Mark Bradley				"Faster, faster, until the thrill of
Manager I/O Subsystems			 speed overcomes the fear of death."
Silicon Graphics Computer Systems
Mountain View, CA			     ---Hunter S. Thompson