RAM disk vs paging + buffer cache

[#Bill_Stewart]

In article <388 at rtech.UUCP> daveb at rtech.UUCP (Dave Brower) writes:
>In article <514 at opus.nbires.UUCP> rcd at nbires.UUCP (Dick Dunn) writes:
>>What generally-useful features does a RAM disk have that I didn't consider?
> [.............]  It's conceivable to build a BIIG ram disk for cheap,
>and be nearly vendor independent.
In fact, it's been done.  One of the companies that sells off-brand
memory for VAXen (Dataram?) also sells a RAM disk-emulator for UNIBUS
and probably some other busses.  Prices were comparable to regular
memory, with some overhead for the initial equipment.
>Imagine a 160M Eagle plug compatible ram disk with 'horribly' slow 450us
>dynamic RAM used as the paging area, /tmp, and /usr/tmp.  That helps you
>right where some of the biggest bottlenecks are located.
What I'd like to see would be a ram disk that overflowed onto magnetic
disk - runs from ram if possible, and puts blocks onto magnetic disk in
some sort of LRU fashion if necessary.  One possible variant on this is
to allow file systems to span multiple physical media, and to use a ram
disk as one part.  This could be valuable in its own right.
-- 
# Bill Stewart, AT&T Bell Labs 2G-202, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs