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

Types of Swap Space
There are three types of swap space used for paging operations:
• Device Swap
• File System Swap
• Pseudo Swap
Device Swap
Swap space is initially allocated when you configure your disks. Device swap space occupies a
logical volume or disk partition that is typically reserved expressly for paging purposes. This
space may also be configured as a dump area but doing so has implications for memory dump
integrity if a crash occurs. See “Using a Device for Both Paging and Dumping (System Recovery
Time)” (page 82). Because, when HP-UX is running, the device is used exclusively for paging,
you cannot also store files on it.
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.
60 Major Components of HP-UX