RAM disk.

[Malcolm Weir]

OK, Reach For Your Revolver... Make My Day! But you dudes who say "RAM disks?
Unnecessary, 'cos we've got a Buffer Cache!!" are WRONG, INCORRECT, MISTAKEN,
and basically WAY OUT OF LINE.

Consider the AT&T System Administrator's Guide section on "Performance Tuning".
This source will indicate that it is a Good Thing to put heavily used
directories in the physically-centered cylinders of your faster disks. Obvious,
huh? OK, now stuff the same data on a RAM disk, as a logical extension.
"But", you cry, "the Buffer Cache would do that for us!"

Really? just how do you persuade *nix to cache "/lib/*", in prefence to 
Joe Unimportant-User's huge statistical jobs that have been munging vast
amounts of data for the past 12 days?  How do you persuade it that the
disk accesses caused by the backup of "/irrelevant" are less important than
the accesses caused by the CEO's secretary's WP temp files?

There are MANY applications where RAM disk provide incredible benefits.
Don't disparage them just because you don't know how REAL systems are used,
with varying importance of tasks.

A perfect example of a manufacturer who has done excellent things with RAM
disk is Pyramid Technology. The allow all sorts of neat things to do with
RAM disk, for example:

    * Mirror a RAM disk to a physical disk. The data is *always* in the
      cache, yet will eventually be on magnetic. This does wonderful
      things if you stick database dictionaries/upper-levels on it.

    * Define a logical slice consisting of a RAM disk followed by
      magnetic. The idea being that you get a scratch space that 
      is esp. fast, and has an economic price, but can accomodate
      periodic heavy demands.

Disclaimer: I have nothing to do with Pyramid. In fact, Sanyo/Icon
computers do not support RAM disk, but we do support huge buffer
caches, with a dedicated processor to keep track of whats where.

Of course, RAM disks made from Virtual Memory are truly Stoopid, but
there are even a couple of manufacturers out there who build real RAM
disks: 5 1/4" form factor enclosures, SCSI interface (or whatever), but
huge quantities of DRAM instead of magnetics. The De Luxe versions even
have a battery...  how do these relate to Buffer Cache?

(btw, I used to be anti-RAM-disk, 'till I tried a system with "/lib"
 on RAM. "/tmp" didn't make that much difference, but you should've
 seen "ld" fly... )

Malc.