Installation guide

label with the disklabel command. Refer to Section 7.8 and to the
disklabel
(8) reference page for more information.
7.1.2 Adding Swap Space
The Tru64 UNIX operating system uses a combination of physical memory
and disk space to create virtual memory, which can be much larger than
the physical memory. Virtual memory can support more processes than the
physical memory alone. This section and the sections that follow describe
important virtual memory concepts that you should consider when
configuring swap space.
The basic unit of virtual memory and physical memory is the page. Virtual
memory attempts to keep a process’ most recently referenced virtual pages
in physical memory. When a process references virtual pages, they are
brought into physical memory from their storage locations on disk.
Modified virtual pages can be moved to a temporary location on the disk
called swap space if the physical pages (the pages in physical memory) that
contain the virtual pages are needed by either a newly referenced virtual
page or by a page with a higher priority. Therefore, a process’ virtual
address space can consist of pages that are located in physical memory,
stored temporarily in swap space, and stored permanently on disk in
executable or data files. The operating system uses two operations to move
virtual pages between physical memory and disk: paging and swapping.
Paging involves moving a single virtual page or a small cluster of pages
between disk and physical memory. If a process references a virtual page
that is not in physical memory, the operating system reads a copy of the
virtual page from its permanent location on disk or from swap space into
physical memory. This operation is called a pagein. Pageins typically occur
when a process executes a new image and references locations in the
executable image that have not been referenced before.
If a physical page is needed to hold a newly referenced virtual page or a
page with a higher priority, the operating system writes a modified virtual
page (or a small cluster of pages) that has not been recently referenced to
the swap space. This operation is called modified page writing or a pageout.
Note that only modified virtual pages are written to swap space because
there is always a copy of the unmodified pages in their permanent locations
on disk.
Swapping involves moving a large number of virtual pages between
physical memory and disk. The operating system requires a certain amount
of physical memory for efficient operation. If the number of free physical
pages drops below the system-defined limit, and if the system is unable to
reclaim enough physical memory by paging out individual virtual pages or
7–4 Administering the UNIX File System