Oracle Solaris OS software and application software can use some disk space for temporary storage rather than for file systems. The reserved area of the disk is called swap space. Swap space is used as virtual memory storage areas when the system does not have enough physical memory to handle current processes. In a ZFS root file system, the disk space reserved for swap is a ZFS volume.
The virtual memory system maps physical copies of files on disk to virtual addresses in memory. Physical memory pages that contain the data for these mappings can be backed by regular files in the file system, or by swap space. If the memory is backed by swap space it is referred to as anonymous memory because no identity is assigned to the disk space that is backing the memory.
The Oracle Solaris OS uses the concept of virtual swap space, a layer between anonymous memory pages and the physical storage (or disk-backed swap space) that actually back these pages. A system's virtual swap space is equal to the sum of all its physical (disk-backed) swap space plus a portion of the currently available physical memory.
Virtual swap space has these advantages:
The need for large amounts of physical swap space is reduced because virtual swap space does not necessarily correspond to physical (disk) storage.
A pseudo file system called SWAPFS provides addresses for anonymous memory pages. Because SWAPFS controls the allocation of memory pages, it has greater flexibility in deciding what happens to a page. For example, SWAPFS might change the page's requirements for disk-backed swap storage.