Disk Xfer Rates vs Bus Speed

[markb at denali]

In article <1988Oct12.164433.17763 at utzoo.uucp>, henry at utzoo.uucp (Henry Spencer) writes:
> Uh, say what?  SCSI transfer rate, at full bore, is 4 MB/s (note, not
> Mb/s), which is 32 MHz.  Sounds comparable to me.  Of course, there are
> a lot of cruddy SCSI controllers which can't hack that kind of speed,
> but then, there are cruddy ESDI and SMDE controllers too.
> 
Yes, bus speed is at 4 MB/s burst speed.  The arbitration cycles are
spec'ed in milliseconds.  Read the latest SCSI 1 spec for actual numbers.
The ESDI bus speeds are in microseconds.  The ESDI spec provides actual
figures, again.  These are industry standards.  Please read them.
Look especially at the timing diagrams.
 
> References, please.  I've seen both SCSI and ESDI specs, and somehow I
> failed to notice any such disparity.  In the specs, not the current
> (often lousy, for both) implementations.
> 
> Do remember that this is, to some extent, an apples-and-oranges comparison,
> since SMDE and ESDI are drive-to-controller interfaces and SCSI is a
> controller-to-host interface.  Since there *has* to be a controller
> between a disk drive and a SCSI bus, the quality of the controller makes
> a big difference.

A bus is a bus.  Look at the data rate of data from the media, then at the
bus timings, and it becomes very clear that unless you are doing some hefty
read-aheads and intelligent buffer managemant, that ESDI does blow it away,
both on paper, and in practice.  Better yet, run a benchmark that requires
substantial paging to disk (ie. memory limited) and see which drive types
do it faster... 

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