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Full Discussion: How to Switch off a Unix machine abruptly
Top Forums UNIX for Advanced & Expert Users How to Switch off a Unix machine abruptly Post 302679739 by bakunin on Tuesday 31st of July 2012 02:27:39 PM
Old 07-31-2012
One of the side results of some in-depth HACMP-tests we (some colleagues of mine and me) carried out 15 years ago was a collection of more or less ingenious ways to stop a system as fast as possible. Since then i have tested now and then these methods - more out of curiosity than for any practical purpose. These are the methods we found, listed in order from slow and safe to fast and dangerous:
  1. shutdown -h now

  2. shutdown -fh now
    Some systems use "-F" instead of "-f". Is supposed to skip some safety measures and therefore to be faster.

  3. sync ; sync ; halt
    will be even faster but might (only in extreme cases) damage the system because it stops without anything being closed in order. Still, most systems will survive this and boot more or less properly after being killed that way.

  4. halt -q
    Even faster, but don't try this without a journaling file system (don't try this at all without the need to do so). Old SunOS versions, for instance, had a fair chance of not coming up properly again after such a nosedive. Even with a journaling FS there is some risk, which is why it should only be used as either a very extreme measure or for testing purposes on expendable (test) systems. The same goes for the "winner":

  5. cat /etc/hosts > /dev/kmem
    Any other file will do too - i use this just out of habit. On POWER and PC hardware (and probably on most other hardware too) this was consistently faster than even "halt -q", it takes only about a dozen bytes to be written over the kernel image to crash the system. In AIX this is not possible any more, because even root cannot write to /dev/kmem in newer versions.

I hope this helps.

bakunin
 

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LEARN ABOUT V7

halt

halt(1M)						  System Administration Commands						  halt(1M)

NAME

       halt, poweroff - stop the processor

SYNOPSIS

       /usr/sbin/halt [-dlnqy]

       /usr/sbin/poweroff [-dlnqy]

DESCRIPTION

       The  halt  and  poweroff  utilities  write  any	pending information to the disks and then stop the processor. The poweroff utility has the
       machine remove power, if possible.

       The halt and poweroff utilities normally log the system shutdown to the system log daemon, syslogd(1M), and place a shutdown record in  the
       login accounting file /var/adm/wtmpx. These actions are inhibited if the -n or -q options are present.

OPTIONS

       The following options are supported:

       -d	Force a system crash dump before rebooting. See dumpadm(1M) for information on configuring system crash dumps.

       -l	Suppress sending a message to the system log daemon, syslogd(1M), about who executed halt.

       -n	Prevent the sync(1M) before stopping.

       -q	Quick halt. No graceful shutdown is attempted.

       -y	Halt the system, even from a dialup terminal.

FILES

       /var/adm/wtmpx	       History of user access and administration information.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       dumpadm(1M), init(1M), reboot(1M), shutdown(1M), sync(1M), syslogd(1M), inittab(4), attributes(5), smf(5)

NOTES

       The halt and poweroff utilities do not cleanly shutdown smf(5) services. Execute the scripts in /etc/rcnum.d or execute shutdown actions in
       inittab(4). To ensure a complete shutdown of system services, use shutdown(1M) or init(1M) to reboot a Solaris system.

SunOS 5.10							    2 Nov 2004								  halt(1M)
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