query: new disk drive for VAX

[Doug Alan]

>> [Doug Alan:] Why spend big bux on SuperEagles when you can get 800
>> Meg, high speed |5.25-inch Winchesters these days for $3,300 each?

> [Chuck Seeger:] Performance, of course.  The new SMD drives and
> controllers offer 3.0 MBps interfaces (24 Mbps).  I belive that
> these 5.25" drives are limited to about 10 Mbps.  This is important
> on modern fast machines, but the VAX in question may be too old and
> slow to qualify.

Well, I'm not sure of this "of course".  Our ESDI controllers (Sigma
SCD-RQD11-EC's) certainly provides 24 Mbps also.  The 5.25-inch disk
drives we use (Maxtor XT8760E's) only do 15 Mbps, but if we multiply the
number of sectors per track (52) by the number of rotations per second
(60) we find that the disk drive can only get data off the disk at
12.5 Mbps, so having a higher transfer rate would probably have no
effect.  Future ESDI drives that have more sectors per track, will
have higher transfer rates.

In order for an SMD disk to provide much better performance, it will
have to either spin much faster, have many more sectors per track, or
have a much smaller seek time (or a less emphatic combination of the
above).  In no way is SMD inately superior to ESDI.  (Except that SMD
allows much longer cable distances.)

On the other hand, the 5.25-inch drives are also available in SCSI
versions.  SCSI offers potentially higher performance and versatility
than either SMD or SCSI.  The only reason we didn't go with SCSI is
because we already have a base of ESDI here.

|>oug /\lan
   (or nessus at athena.mit.edu
       nessus at mit-eddie.uucp)