HP-UX System Administrator's Guide: Overview HP-UX 11i v3 (B3921-90011, September 2010)

NOTE: The preceding 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 very small.
Using a Device for Both Paging and Dumping (Crash Integrity)
It is possible to use a specific device for both paging purposes and as a dump device. But, if crash
dump integrity is critical to you, this is not recommended. From the savecrash(1M) manpage:
“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 will indicate that
the dump may be invalid. If a dump device has not already been enabled for paging,
savecrash prevents paging from being enabled to the device by creating the file /etc/
savecore.LCK. swapon does not enable the device for paging if the device is locked in
/etc/savecore.LCK.”
So, if possible, avoid using a given device for both paging and dumping: particularly the primary
paging device!
HP-UX systems configured with small amounts of memory and using only the primary swap
device as a dump device are in danger of not being able to preserve the dump (copy it to the
HP-UX file system area) before paging activity destroys the data in the dump area. HP-UX
systems configured with larger amounts of memory are less likely to need paging (swap) space
during startup, and are therefore less likely to destroy a memory dump on the primary paging
device before it can be copied.
Disk Space Needs
Use this section if you have very limited disk resources for the post-crash dump and/or the
post-reboot save of the memory image to the HP-UX file system area. The factors you have to
consider here are:
Dump Level
Compressed Save versus Non-compressed Save
Partial Save (savecrash -p)
Dump Level
You are reading this section because disk space is a limited resource on your server. Obviously,
the fewer pages that you have to dump, the less space is required to hold them. Therefore, unless
your server also has a small amount of physical memory, a full dump is not recommended. If
disk space is very limited, you can always choose no dump at all.
However, there is a happy medium, and it happens to be the default dump behavior, which is
called a selective dump. HP-UX does a pretty good job of determining which pages of memory
are the most critical for a given type of crash, and saves only those. By choosing this option, you
can save a lot of disk space on your dump devices, and again later, in your HP-UX file system
area. For instructions on how to do this, see HP-UX System Administrator’s Guide: Routine
Management Tasks.
Compressed Save versus Non-compressed Save
Regardless of whether you choose to do a full or selective save, whatever is saved on the dump
devices usually needs to be copied to your HP-UX file system area before you can use it.
If the disk space shortage on your system is in the HP-UX file system area (not in the dump
devices), you can choose to have savecrash (the boot time utility that does the copy) compress
your data as it makes the copy.
84 Major Components of HP-UX