RAM disk vs paging + buffer cache

[rbl at nitrex.UUCP]

<Line-eater's lunch>

About 12 years ago, I developed a solid-state disk marketed by Monolithic
Systems Corp. of Englewood CO. (Known as EMU for 'extended memory unit'.)
One of my graduate students at Case Western Reserve did a dissertation on
UNIX performance enhancements using this disk  ---  which works much like
a RAM disk.  Turned out that one of the best strategies was to have TWO
disks (since a dual-ported version was available, one could have one unit
partitioned and two controllers).  One partition was for /usr/tmp and the
other was to hold the root.  Idea was that frequently used system programs
would have greatly reduced latency and that the user's scratch space would
be similarly sped up.

Note:  if a device driver is required, much of the inherent speed advantage
is lost.  Drivers consume milliseconds per access, not very noticible when
disk latency is tens to hundreds of milliseconds.  When RAM is accessed,
rotational and seek latency go away and the driver delays are very noticable.

By the way, the solid-state disk enabled us to do some real-time applications
of UNIX very nicely.  A 1 million sample/sec analog-to-digital converter was
one of these.

Robin B. Lake
Standard Oil Research and Development
4440 Warrensville Center Road
Cleveland, OH  44128
(216)-581-5976
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