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dump driver sysdump crash kernel

From: Bela Lubkin <belal@sco.com>
Subject: Re: Enabling the "dump driver" - how to?
Date: Mon, 21 Jan 2002 09:14:05 GMT
References: <5.0.0.25.2.20020120145451.027532a0@172.20.1.1>
<3C4B334C.1010600@aplawrence.com>
<5.0.0.25.2.20020120164507.027451e0@172.20.1.1> 

Ken Wolff & Tony Lawrence wrote:

Ken>>> Running OSR506 I'm trying to run crash against a dumpfile but get the 
Ken>>> fillowing error:
Ken>>> crash: Cannot mmap dumpfile dumpfile: Please enable the `dump' driver Y 
Ken>>> (error 19)
Ken>>> Does anyone know how I enable the 'dump' driver?

Tony>> Most likely by simply setting its /etc/conf/sdevice.d file to contain a Y 
Tony>> at the place it now says N and relinking.

Ken> Thanks Tony.  Is this something I've done before.....since I have been able 
Ken> to run crash under 504 and 505?  Also, will relinking the kernel trash the 
Ken> symbols crash needs to look at this dumpfile since it was performed under 
Ken> the old kernel?  Should I have the current kernel to look at this dump?

The handling of crash dumps has changed considerably in 5.0.6.
Previously, dumpsave(ADM) was a script that used dd(C) to save a raw
image of a dump.  That raw image was useless without the corresponding
kernel.

In 5.0.6, dumpsave uses sysdump(ADM).  One of sysdump's properties is
that it merges together the kernel and memory image into a single
self-contained file.  You can run crash(ADM) or scodb(ADM) on such a
file without specifying a separate kernel image.

sysdump first shipped with 5.0.4, but dumpsave wasn't modified to use it
until 5.0.6; also, all of these utilities have been incrementally
improved over time.  The 5.0.6 versions of crash, scodb and sysdump are
backwards compatible at least to 5.0.0.  I'm not so sure about the
dumpsave script; it might or might not be.

Besides just being a better way of doing things, one primary reason
dumpsave was changed to use sysdump is that sysdump can edit the dump in
various ways to make it smaller.  Previously there was no particularly
good way to save a dump of a system with >2GB RAM, because OSR5 doesn't
support regular files >2GB.  You couldn't just use dd to make a raw
dump, you'd exceed the file size limit.  sysdump automatically omits the
least useful items (unused pages of memory, then pages that belong
purely to user processes) in order to keep the full dump smaller than
2GB.  So 5.0.6's dumpsave script works on >2GB machines.

sysdump can also create files in two output formats.  With its "-O" flag
it creates images that look just like a raw crash dump with a symbol
table added.  crash and scodb can operate directly on such a file.  But
such files are at least as large as the original system's memory.  If
the original system's memory is >2GB, a "-O" image can only be written
to a block device (like /dev/swap).

With sysdump's "-o" flag, it actually omits pages that are omitted from
the image.  That is, with "-O", when there is an unused page, it writes
4K of 0s; with "-o", it writes nothing.  An index somewhere in the file
tells how to find a particular page.  (Imagine a censored book: with -O,
the censor has eliminated the words on some of the pages, but left them
in the book as blank pages.  With -o, he's actually ripped those pages
right out; the final product is much thinner than the original book.)

What you ran into was a "-o" formatted dump.  crash & scodb can't
operate directly on it, but _can_ operate on it with the assistance of
the "dump" driver.

You can also access such dumps without linking in the "dump" driver, by
expanding them:

  sysdump -i dump -O dump.expanded
  crash -d dump.expanded
  scodb -d dump.expanded

If it was a dump of >2GB, you would have to use a block device like
/dev/swap instead of a file like "dump.expanded".

BTW, the driver that helps crash & scodb understand compacted dumps is
called "dump".  It should not be confused with another driver,
"sysdump", which is what actually _writes_ crash dumps during a panic.
And the _driver_ "sysdump" should not be confused with the _utility_
sysdump(ADM) (/etc/sysdump), which is the user-level utility for copying
(and modifying which types of pages are saved in) crash dumps.

>Bela<
 

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