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A temporary form of data storage is swap space. It should be noted that HP-UX does not "swap" any more, it pages and, as a "last resort" deactivates processes. The process of deactivation replaces what was formerly known as swapping entire processes out.

While executing a program, data and instructions can be paged (copied) to and from secondary storage, if the system load warrants.

Swap space is initially allocated when the system is configured. HP-UX supports two types of swap space: device swap and file system swap. Device swap is allocated on a disk "outside" of any file system space and can be:

  1. A entire disk
  2. An area on a disk
  3. A logical part of a physical disk

If the entire disk hasn't been designated as swap, the remaining space on the disk can be used for a file system. File system swap space is allocated from a mounted file system and can be added dynamically to a running system. Device swap can also be added dynamically to a running system. Sam or the swapon() command can be used to enable device or file system swap.

NOTE: File-system swap has significantly lower performance than device swap. The I/O for file system swap will contend with user I/O on that particular file system. Using file system swap space should be avoided. Once allocated, you cannot remove either type of swap without rebooting the system. HP-UX uses a swap space reservation method (to insure it has space available), but only allocates the space when it actually needs to write to it.