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Recovering from failures
Abnormal system shutdowns
Dump definitions built into the kernel vs. defined at
runtime
There are three places to define which devices are to be used as dump
devices:
1. During kernel configuration
2. At boot time (entries defined in the /etc/fstab file)
3. At run time (using the /sbin/crashconf command)
Definitions at each of these places add to or replace any previous
definitions from other sources. However, consider the following situation:
Example
A system called appserver has 1-Gbyte of physical memory. If the dump
devices for this system are defined with a total of 256-Mbytes of space in
the kernel file and an additional 768-Mbytes of disk space in the /etc/
fstab file, there would be enough dump space to hold the entire memory
image (a full dump).
If the crash occurs, however, before /etc/fstab is processed, only the
amount of dump space already configured is available at the time of the
crash; in this example, it is 256-Mbytes of space.
Define enough dump space in the kernel configuration if it is critical to
capture every byte of memory in all instances, including the early stages
of the boot process.
NOTE
This example is presented for completeness. The actual amount of time
between the point where kernel dump devices are activated and the
point where runtime dump devices are activated is very small (a few
seconds), so the window of vulnerability for this situation is practically
nonexistent.
Using a device for both paging and as a dump device
It is possible to use a specific device for both paging purposes and as a
dump device. If, however, crash dump integrity is critical, this is not
recommended.
If savecrash determines that a dump device is already enabled for
paging and that paging activity has already taken place on that device, a
warning message indicates that the dump may be invalid. If a dump
device has not already been enabled for paging, savecrash prevents
Chapter 7
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