Path: utzoo!utgpu!attcan!uunet!ficc!karl
From: karl@ficc.uu.net (karl lehenbauer)
Newsgroups: comp.arch
Subject: Re: Solid State Secondary Storage
Summary: don't page to RAM unless you can't add directly-addressed RAM
Keywords: ram, wafer, disk
Message-ID: <2710@ficc.uu.net>
Date: 12 Jan 89 17:42:09 GMT
References: <248@vlsi.ll.mit.edu> <408@laic.UUCP>
Distribution: comp
Organization: Ferranti International Controls
Lines: 29

In article <408@laic.UUCP>, darin@nova.laic.uucp (Darin Johnson) writes:
> It would make a very nice paging device.  Since decent paging devices
> are relatively expensive (we're talking fast disks, not SCSI or
> ST225's), this would be a nice alternative.  Think about something like
> common LISP on a personal computer.  Currently, most do not have paging,
> and if they did have paging it would be to a slow disk.  A device like
> you described would vastly improve the performance.

Unless you already had as much directly addressed memory as your bus could
support, it would always be a win under a VM system to add RAM as
bus memory rather than as a fast disk for paging use, because you'll still
take page faults to get data from your RAM disk but you won't for the same
data in directly addressed RAM.  

Also, support for mapped files is starting to show up (Mach, etc), the
result being that your files logically appear in your address space and
data is loaded from page faults.  This makes having lots of directly
addressed RAM all the more desirable.

RAMdisks have themselves been around for a long time.  I had one on my Apple
II.  Better, I think, is to use the RAM (directly addressed or bank switched)
as a cache.  That way you don't have to make the decisions of what to copy
into RAM disk nor do you have to remember to copy it out after it's been
updated.  DEC used to sell a PDP-11 RAM disk a long time ago called EM-11,
"non-rotating mass storage."
-- 
-- uunet!ficc!karl	"The greatest dangers to liberty lurk in insidious
-- karl@ficc.uu.net	encroachment by men of zeal, well-meaning but without 
			understanding." -- Justice Louis O. Brandeis