Semi-conductor Disk for VAX

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

From:      Mike Muuss <mike at brl-vgr>

Howard -

BRL is presently operating two VAXen with the new (>= 4.1c) filesystem
using a "solid-state disk" device.  In both cases the device is
a DATARAM BULK MOS, with 8 Mbytes of memory.

Discussion:

Using this sort of device for swapping/paging is REDICULOUS.
Your money is much better spent mererly adding more memory to
your VAX.  8 Mbytes is about right for a 780, 6 Mbytes for a 750.
With that quantity of memory, paging is a pretty low-level activity
unless you are running ultra-huge problems.

Using this device for /tmp can be somewhat worthwhile, because
things like ED and CC do lots of SEQUENTIAL I/O on largish files
in /tmp;  hence no amount of physical memory devoted to the UNIX
buffer cache is likely to be able to cache the whole thing.

However, the current cost/performance tradeoffs must be carefully
examined.  My present feeling is that it would be FAR better to
invest another ~$14K in another 9766 style disk, and JUST use
the first few Mbytes for /tmp, RATHER than spending ~25K for
a BULK MOS.  Also, you can hang your 9766 off the SBI or CMI,
whereas the BULK MOS units I am familiar with can usually only
be attached to the UNIBUS (at least on VAXen.  There are 11/70
Cachebus devices availible).

Now, if the machine were a PDP-11 instead of a VAX, then things
would be entirely different, and I would whole-hartedly recommend
getting a BULK MOS unit.  Every 11/70 at BRL has at least 1 such
device, and one has two BULK memory units, and they make a *tremendous*
performance improvement.  But VAXes can take oodles of physical memory.
Why hide it behind an interface bus?

		-Mike