dontdump(5)

NAME

dontdump — defines which classes of kernel memory pages are not dumped when a kernel panic occurs

VALUES

Failsafe

0

Default

0 (Allow the kernel to choose which classes to dump.)

Allowed values

Integer values from 0 to 1024.

The integer value should be the sum of the integer values for the excluded classes as follows:

UNUSED: 2 : Unused pages
USERPG: 4 : User pages
BCACHE: 8 : Buffer cache
KCODE: 16 : Kernel text pages
USTACK: 32 : Process stack
FSDATA: 64 : File-system Metadata
KDDATA: 128 : Kernel dynamic data
KSDATA: 256 : Kernel static data
SUPERPG: 512 : Unused superpage pool

Recommended values

0 (Allow the kernel to choose which classes to dump.)

The value the kernel usually chooses is 30 = UNUSED + USERPG + KCODE + BCACHE. The kernel chooses these classes because they are not usually useful in debugging kernel problems.

DESCRIPTION

On large systems, the time required to dump system memory when a kernel panic occurs can be excessive or even prohibitive, depending on how much physical memory is installed in the system. Fast-dump capabilities controlled by the dontdump and alwaysdump parameters provide a means for restricting kernel dumps to specific types of information:

. Unused Physical Memory
. User Process
. Buffer Cache
. Kernel Code
. Process Stack
. File-System Metadata
. Kernel Dynamic Data
. Kernel Static Data
. Unused Superpage Pool

The crashconf command, and its associated configuration file /etc/rc.config.d/crashconf, control which of these memory classes are to be included in the memory dumps associated with a kernel panic. On rare occasions, the system may panic before crashconf(1M) is run during the boot process. On those occasions, the configuration can be set using the alwaysdump and dontdump tunables.

The bit-map value stored in dontdump specifies which of these memory classes are not to be included in the memory dumps associated with a kernel panic.

The default value for this parameter is 0. The system determines whether or not to dump classes of memory based on the type of crash that occurs, in this case.

Note that certain types of system crash, require a full crash dump. Also, the system operator may request a full crash dump at the time the dump is taken. In either of these cases, a full dump will be performed regardless of the classes selected using dontdump.

Who Is Expected to Change This Tunable?

Only HP Field engineers should change the value of this tunable.

Restrictions on Changing

Changes to this tunable take effect at next reboot. Use crashconf to change the selection of pages to take effect immediately.

When Should the Tunable Be Turned On?

This tunable should be turned on to exclude certain classes of pages from being dumped when there is a system crash. This can be done to speedup the dump.

What Are the Side Effects of Turning the Tunable On?

If pages necessary for analyzing the dump are excluded, then the dump may not be useful for identifying the cause of the system crash.

When Should the Tunable Be Turned Off?

The tunable is turned off by default.

What Are the Side Effects of Turning the Tunable Off?

The system decides the page classes that must not be dumped based on the type of crash. The dump may take longer.

What Other Tunables Should Be Changed at the Same Time?

The alwaysdump tunable should not contain the same page classes as dontdump.

WARNINGS

All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

dontdump was developed by HP.