HP-UX System Administrator's Guide: Overview

By reducing the time required to store the entire dump the recovery period is shorter
and your system can be returned to service much sooner. Dump compression provides
a greater time saving on systems that have large amounts of memory.
• Dump compression is not forced, it is only a user request that will be honored if
possible.
At the time of a system crash the dump subsystem examines the state of the system
and its resources to determine whether it is possible to use compression. Depending
on the resources available, HP-UX determines dynamically whether to dump using
compressed or uncompressed format.
(For example if the processor that is processing the crash fails to assign a sufficient
number of processors to do the compression, the dump will not be compressed.
A recursive crash, such as a panic during the processing of a previous dump, also
causes the system to dump using uncompressed format.)
• For selective dumps that exclude unused pages, you can expect the dump to take
about one-third the time of uncompressed dumps on the same server. This interval
includes the time required to run the savecrash program and write the dump
to its final storage location on the HP-UX file system. A dump that previously took
3 hours to complete should now take only about an hour.
• You can use the crashconf command (see crashconf(1M)) to disable or enable
compressed dumps. (Compression is configured into the kernel by default.) During
a crash event you can also choose to override the previously defined dump
compression setting.
Normally, there is no benefit in disabling compression unless the initial
(compressed) dump is corrupt and you want to attempt an uncompressed dump
on a subsequent crash event. Compressed saves (to the HP-UX file system area)
are only possible with sequential dumps.
• Using the command crashutil, you can convert the compressed dump file to
any of several dump formats for storage and analysis. See the manpage crashutil(1M)
for detailed information on how to do this and what dump formats are available.
• A compressed dump file requires less disk storage space and creates a smaller tar
file that takes less time to copy to tape or transmit for analysis, for example via
ftp)
• If your server uses virtual partitions (vPars), the dump might not be compressed
but the dump process will proceed.
• If more than one crash occurs in close succession, it might not be possible for
HP-UX to compress the dump.
Compressed Save versus Noncompressed Save
System dumps can be very large, so large that your ability to store them in your HP-UX
file system area can be taxed.
98 Major Components of HP-UX