RAM disk vs paging + buffer cache

[Karl Kleinpaste]

judah at whuxcc.UUCP (Judah Greenblatt) writes some very interesting
comments on Dick Dunn's remarks on RAM discs.
>I was involved in developing ram-disk drivers for several unix systems
>in an attempt to speed up a large application.  During the work we
>discovered the following useful tidbits:
>...
>-  To play these games requires a LOT of memory.  Don't even think
>   of allocating less than 1 MB to a ram disk.  If you will be putting
>   programs into core, expect to use 10 MB or more.  As you will also
>   want 1000 or more block buffers, and probably need to support 100+
>   users, you're going to need at least 32 MB of ram.

That depends on what the system is and what it's doing.  I use
PDP-11/73s running SysV with 4Mb and (sigh) 4 RL02-equivalent drives
with quite some frequency.  The result without RAM discs was pretty
positively miserable overall system throughput.  For example,
compiling a complete kernel from scratch took roughly 45 minutes, and
that's with just me on it alone.  I added a set of small RAM discs to
it, occupying about 1Mb total, 512K of which was /tmp, 256Kb on
/usr/tmp, and that complete kernel compilation dropped to barely 30
minutes.  For my applications (which involved lots of recompilation of
lots of things), the smaller RAM discs are quite useful.
-- 
Karl Kleinpaste