Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucbcad!ucbvax!FNALB.BITNET!WARNER..NAGY
From: WARNER..NAGY@FNALB.BITNET (Frank.J.Nagy@jade.Berkeley.EDU,
        VAX Wizard & Guru)
Newsgroups: comp.os.vms
Subject: RE: THE DUMP File     -- Uses and abuses
Message-ID: <8712250121.AA26559@jade.berkeley.edu>
Date: 23 Dec 87 19:54:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 45

Richard Steinberger <STEINBERGER@KL.SRI.COM> writes:

> There is a dump file on my system disk that is 32767 blocks large (the
> same size as physical memory).  My understanding is that this area will
> be used by VMS in case the system crashes.  The contents of memory will
> be stored here.  This is fine; but I was wondering if there are any ways
> to have this file/disk area serve two functions since it seems to be
> unused during "normal" VMS operations.  For example, is there a way that
> it can also be a page or swap file until/unless the system crashes?
> Are there other uses or should I just keep my hands off?  Thanks
> in advance to all who respond.

To make a full system save after a crash, the dump file should be your
physical memory size (in pages) + 4 blocks (1 for header and 3 for error
log buffers, I believe).  If the dump file is shorter, VMS will just not
write all of physical memory out which just means that SDA will probably
not be able to analyze the system crash.  So, if disk space is *much*
more important than analyzing a system crash (which is true of some
systems), then you could either create a 0 length dump file (REBOOT before
doing a PURGE or you'll be sorry).

Alternatively, I believe# if there is no SYSDUMP.DMP file, the crash dump
will be saved in the pagefile.  The system parameter SAVEDUMP will then
preserve the portion of the pagefile with the crash dump until SDA is
used to copy the crash dump to another file for analysis.  Because this
*reserves* pagefile space, your pagefile must be larger than physical
memory and the crash dump should be emptied out of the pagefile before
startup is completed to release the pagefile space.

#This is an unverified statement, maybe someone else in netland can respond
 as to what circumstances are needed to have VMS use the pagefile for
 system dumps.

We have debugging systems which have a normal pagefile setup, other systems
with a 0 block pagefile (if the system crashes there, we depend upon being
able to reproduce it on a debug station), and VAXClusters (CI and LAVC)
with a single SYSDUMP.DMP in SYS$COMMON:[SYSEXE].  The *common* dumpfile
is only good if a single VAX crashes, but certainly saves disk space in
clusters with 128 MB 8800's and 60 MB 8650's!

= Frank J. Nagy   "VAX Guru & Wizard"
= Fermilab Research Division EED/Controls
= HEPNET: WARNER::NAGY (43198::NAGY) or FNAL::NAGY (43009::NAGY)
= BitNet: NAGY@FNAL
= USnail: Fermilab POB 500 MS/220 Batavia, IL 60510