Persistent versus working segments

Persistent segments are permanent while working segments are temporary.

The pages of a persistent segment have permanent storage locations on disk. Files containing data or executable programs are mapped to persistent segments. Because each page of a persistent segment has a permanent disk storage location, the VMM writes the page back to that location when the page has been changed and can no longer be kept in real memory. If the page has not changed when selected for placement on a free list, no I/O is required. If the page is referenced again later, a new copy is read in from its permanent disk-storage location.

Working segments are transitory, exist only during their use by a process, and have no permanent disk-storage location. Process stack and data regions are mapped to working segments, as are the kernel text segment, the kernel-extension text segments, as well as the shared-library text and data segments. Pages of working segments must also have disk-storage locations to occupy when they cannot be kept in real memory. The disk-paging space is used for this purpose.

The following illustration shows the relationship between some of the types of segments and the locations of their pages on disk. It also shows the actual (arbitrary) locations of the pages when they are in real memory.

Figure 1. Persistent and Working Storage Segments. This illustration shows the relationship between some of the types of segments and the locations of their pages on disk. It also shows the actual (arbitrary) locations of the pages when they are in real memory. Working segments are transitory, meaning they exist only during their use by a process and have no permanent disk-storage location. Process stack and data regions are mapped to working segments, as are the kernel text segment, the kernel-extension text segments, and the shared-library text and data segments. Pages of working segments must also have disk-storage locations to occupy when they cannot be kept in real memory. The disk-paging space is used for this purpose.

Persistent-segment types are further classified. Client segments are used to map remote files (for example, files that are being accessed through NFS), including remote executable programs. Pages from client segments are saved and restored over the network to their permanent file location, not on the local-disk paging space. Journaled and deferred segments are persistent segments that must be atomically updated. If a page from a journaled or deferred segment is selected to be removed from real memory (paged out), it must be written to disk paging space unless it is in a state that allows it to be committed (written to its permanent file location).