Editing Files-11 (section)

== Disk organization and naming ==
An operational VMS system has access to one or more online disks, each of which contains a complete, independent file system. These are either local storage or, in the case of a cluster, storage shared with remote systems.

[[Image:OpenVMS disks example 1.svg|thumb|Figure 1'': Sample OpenVMS cluster disk configuration]]

In an OpenVMS cluster configuration, non-private disks are shared between all nodes in the cluster ''(see figure 1)''. In this configuration, the two system disks are accessible to both nodes via the network, but the private disk is not shared: it is mounted for use only by a particular user or process on that machine. Access to files across a cluster is managed by the OpenVMS Distributed Lock Manager, an integral part of the file system.

Multiple disks can be combined to form a single large logical disk, or ''volume set''. Disks can also be automatically replicated into ''shadow sets'' for data security or faster read performance.

A disk is identified by either its physical name or (more often) by a user-defined logical name. For example, the boot device (system disk) may have the physical name <kbd>$3$DKA100</kbd>, but it is generally referred to by the [[#Logical names|logical name]] <kbd>SYS$SYSDEVICE</kbd>.

File systems on each disk (with the exception of ODS-1) are hierarchical. A fully specified [[filename]] consists of a nodename, a username and password, a device name, directory, filename, file type, and a version number, in the format:

 NODE"accountname password"::device:[directory.subdirectory]filename.type;ver

For example, <kbd>[DIR1.DIR2.DIR3]FILE.EXT</kbd> refers to the latest version of <kbd>FILE.EXT</kbd>, on the current default disk, in directory <kbd>[DIR1.DIR2.DIR3]</kbd>.

<kbd>DIR1</kbd> is a subdirectory of the master file directory (MFD), or ''root directory'', and <kbd>DIR2</kbd> is a subdirectory of <kbd>DIR1</kbd>. A disk's MFD is identified by <kbd>[000000]</kbd>.

Most parts of the filename can be omitted, in which case they are taken from the current ''default file specification''. The default file specification replaces the concept of "current directory" in other operating systems by providing a set of defaults for node, device name and directory.  All processes have a default file specification which includes disk name and directory, and most VMS file system [[Subroutine|routines]] accept a default file specification which can also include the file type; the <kbd>TYPE</kbd> command, for example, defaults to "<kbd>.LIS</kbd>" as the file type, so the command <kbd>TYPE F</kbd>, with no extension, attempts to open the file <kbd>F.LIS</kbd>.

Every file has a version number, which defaults to 1 if no other versions of the same filename are present (otherwise one higher than the greatest version).  Every time a file is saved, rather than overwriting the existing version, a new file with the same name but an incremented version number is created.  Old versions can be deleted explicitly, with the <kbd>DELETE</kbd> or the <kbd>PURGE</kbd> command, or optionally, older versions of a file can be deleted automatically when the file's ''version limit'' is reached (set by <kbd>SET FILE/VERSION_LIMIT</kbd>). Old versions are thus not overwritten, but are kept on disk and may be retrieved at any time. The architectural limit on version numbers is 32767.  The versioning behavior is easily overridden if it is unwanted.  In particular, files which are directly updated, such as databases, do not create new versions unless explicitly programmed.

ODS-2 is limited to eight levels of subdirectories, and only uppercase, alphanumeric names (plus the underscore, dash, and dollar sign) up to 39.39 characters (39 for the filename and another 39 for the extension). ODS-5 expands the character set to lowercase letters and most other printable ASCII characters, as well as [[International Organization for Standardization|ISO]] [[Latin-1]] and [[Unicode]] characters, increases the maximum filename length and allows unlimited levels of subdirectories. When constructing a pathname for an ODS-5 file which uses characters not allowed under ODS-2, a special "^" syntax is used to preserve backwards compatibility; the file "<kbd>file.tar.gz;1</kbd>" on an ODS-5 disk, for example, would be referred to as "<kbd>file^.tar.gz</kbd>"&mdash;the file's name is "<kbd>file.tar</kbd>", and the extension is "<kbd>.gz</kbd>".