HP-UX System Administrator's Guide: Overview

NOTE: There is one exception to the rule that a given logical volume cannot be used
for both file system space and device swap. If you have unused space between the end
of a file system and the end of the logical volume in which it resides (that is, the file
system is smaller than the logical volume it is in), the unused space (not allocated to
the file system), can be used as device swap space.
Device swap can only be used locally; it cannot be accessed remotely by clients using
network disk access protocols.
Device swap is quickly accessed because HP-UX can get to the logical volume or disk
partition directly to perform large writes or reads.
File System Swap
If the device swap space you have configured on your server is not enough and you
have no more devices that you can dedicate for that device swap space, you can
configure file system swap space.
File system swap allows for extra swap space if there is an occasional need for more than
is allocated as device swap space. It is used only when device swap space is insufficient.
File system swap space is configured as additional swap space to be allocated from
unused space within a file system.
Because file system swap requires the system to perform a greater amount of processing
it is usually slower than device swap and should not be used as a permanent
replacement for a sufficient amount of device swap space. It is best for the occasional
overflow of device swap space.
The file system used for swap can be either a local or a remote file system. Cluster
clients can use remote file system swap for their swap needs. Swapping to a remote
file system is slower than swapping to a local file system and is not encouraged if local
device swap or local file system swap is available.
Pseudo Swap
Pseudo Swap is very different than device swap space or file system swap space. It is
one of the technologies that allow you to more efficiently utilize the resources on your
system.
Pseudo swap “space” does not really exist; HP-UX just behaves as though it has an
extra amount of swap space. Pseudo swap takes advantage of the fact that not all swap
space that is reserved is actually used. This allows a greater number of processes to
run in memory than could be supported by configured swap devices. Pseudo swap is
best used with large memory systems.
If you choose to use the pseudo swap capability (actually, it is enabled by default), an
amount of pseudo swap space equal to 7/8ths of the amount of physical ram available
to your server, nPartition, or virtual partition is used as pseudo swap.
70 Major Components of HP-UX