Semi-conductor Disk for VAX

[kermit%brl-vgr at sri-unix.UUCP]

From:      Chuck Kennedy <kermit at brl-vgr>

I have to agree with both Mike and Ron concerning their thoughts
about using a semi-conductor disk.  In particular, if you plan on
using the bulk MOS for paging, it is a waste of money.  You are
better off buying more memory for your VAX.  Why go through
extra I/O operations to page to a fast Bulk MOS (on the Unibus)
when you can go directly to memory (on the SBI or CMI)?  Especially
when it is cheaper to buy more VAX memory!!  Of course this argument
can only go on for so long before you have to page (but the new
DEC memory controllers for the 780 can handle up to 16MB and you
can have two controllers for a total of 32MB of real memory).
Again, from experience, 8MB on a 780 seems to be about the right
mix for most jobs.  For paging purposes, it still makes sense to
use a 9766 hooked through the SBI or the CMI because of the higher
bandwidth as well as the cost comparison.

Mike's arguments about using the bulk MOS as /tmp echo my own
exactly.  I bought the bulk MOS for BRL-VGR (a VAX/780) hoping
that it might speed things up (especially considering the dramatic
improvements they made on our 11/70s).  I wish now I could trade
the bulk MOS for a couple of 9766 drives.  Having the extra disk
arms really buys a lot of performance.
					Cheers,
						-Chuck Kennedy
						<kermit @ brl>


P.S.  One of these days VGR will turn into a Purdue style dual-VAX
which should almost double the speed of the machine for a mere $60k.
The only holdup is DEC is being so cussedly SLOWWWWW in delivering
a card cage to mount the backplane (you'd think they could just
rip one of the assembly line).  They only took a year to deliver
the CPU spares kit used to build the dual-VAX.