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From: JCV@CERNVM.BITNET.UUCP
Newsgroups: mod.computers.vax
Subject: RE: VMS RAM disk
Message-ID: <8702140431.AA06118@ucbvax.Berkeley.EDU>
Date: Fri, 13-Feb-87 23:31:40 EST
Article-I.D.: ucbvax.8702140431.AA06118
Posted: Fri Feb 13 23:31:40 1987
Date-Received: Sat, 14-Feb-87 14:17:54 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
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Approved: info-vax@sri-kl.arpa

Let's get some things straitened out about the VAX RAM disk which
has been the subject of so much talk lately.
 
This has nothing to do with the usual RAM disks of PCs etc, which have more
memory in RAM than on disk and slow disk access speeds. It would of
course be madness to use such a thing on a VAX during usual operations.
To save disk I/Os on frequently used images, as somebody suggested,
VMS has the feature of global sections (images installed /SHARED);
you can see this effect by looking at the global valid fault rate
(in MONITOR).
 
However, there is one situation in which you would like to be able
to run the system without a system disk: when running standalone backup.
Usually, BACKUP will page to what it thinks is a system disk, but really
is either the console device (floppy or TU58) or, on uVAXes, a TK50.
In latter case, in fact, it just CANNOT page - so a way out has to be found.
 
And this way out consists of using a part of nonpaged pool as a RAM disk,
which is loaded during system startup, and turning off system paging
(via a SYSGEN parameter). The whole mechanism is explained in the comments
contained in - yes, you guessed right - SYS$UPDATE:STABACKIT.COM.
And, well - it even contains in-line the 40-line MACRO programm to
initialize the disk to a specified size of blocks (pool permitting)!
 
So, as you can see, it pays to have a look at the stuff DEC supplies you
with every system!
 
We will try another application of this nice feature in the future:
Every single-disk 750 user has the problem of waiting at least one hour
till standalone BACKUP has started from the TU58s if you want to restore
your disk (to remove fragmentation). It should be possible to build a
TK50-like kit on your disk which you can boot in a few seconds and then
use it to overwrite just that disk!
 
--- Jan Vorbrueggen