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Incomputer data storage, avolume orlogical drive is a distinctly-addressable storage area with a singlefile system.
Storage can be designed and configured in many different and complex ways yet all include the volume concept. A general-purposemass-storage device is typically divided intopartitions with each partition used as a volume with its own file system. A more specialized mass-storage device may be configured forredundancy where a volume spans multiple storage media or physical drives. A smaller storage such as amemory card orfloppy disk might have only one partition or no partition at all (no partition table) such that the volume is logically indistinguishable from the drive. Asimple volume describes the most basic configuration: a volume on onestorage medium with noredundancy orstriping.
The following table describes an example storage configuration for aWindows system with two physical drives where the first drive has two partitions, and the second has only one. Each partition is treated as a volume with its own file system. The drive lettersC:,D:, andE: each identify a volume, a.k.a. logical drive.
| Physical drive | Partition | Filesystem type | Drive letter |
|---|---|---|---|
| D0 | P0 | NTFS | C: |
| P1 | FAT32 | D: | |
| D1 | P0 | FAT32 | E: |
Anoperating system (OS) can potentially recognize a partition without recognizing a volume associated with it, as when a partition has not been formatted for a file system or is using a file system that the OS does not support. This occurs, for example, when Windows encounters a non-native partition, such as theext4 filesystem commonly used withLinux.
A volume can be packed in a single file. Examples include the ISO9660 disc image (CD/DVD image, commonly called "ISO"), and the installer volume for Mac OS X (Apple Disk Image). As these volumes are files in a host volume, they arenot partitions.
The concept of volume applies to any type of storage medium. But, for historical reasons, the termdisk is often used even for non-disk media. For example, the Windows Disk Management utility supports any type of medium.
Although the concept of volume is relatively consistent throughout computing contexts, there are significant differences in implementation and user experience between major categories of operating systems.
In Windows, volumes are handled by the kernel. Volumes and mount points and configured by the user via the Disk Management utility or thediskpart command.
Unlike in a Unix-like system, Microsoft systems do not have a single rootdirectory. The system assigns at least one path to each mounted volume – either adrive letter (shown as a letter followed by acolon) such asF: or a mount point on anNTFS volume having a drive letter, such asC:\Music. In this case, the fileTrack1.mp3 stored in the root directory of the mounted volume could be referred to via pathsF:\Track1.mp3 orC:\Music\Track1.mp3, respectively.
In order to assign a mount point for a volume as a path within another volume, the following criteria must be met:
By default, Windows assigns drive letters to all drives. It reservesA: andB: for floppy disk drives, whether present or not. It usesC: and subsequent letters for all other drives (e.g. SDD, HDD, CD, DVD). The operating system startup drive is most commonly assignedC:, however this is not always the case. Sincepersonal computers now no longer include floppies, andoptical disc and other removable drives typically still start atD:, letters A and B are available for assignment by a user withadministrative privileges. This assignment is used each time a removable volume is inserted as long as various criteria are satisfied including that the removable drive has not been reformatted on another computer (which changes itsvolume serial number).
More than one drive letter can refer to a single volume. This can be configured via theSUBST command.
Removing or changing a drive letter or mount point assignment may break program functionality, as files are not accessible via the same path. For example, if a program is configured to use files in directoryD:\Data and then drive letterD: is unassigned, the program no longer can access its data sinceD:\Data is no longer a valid path.
In Windows Server 2008 and onward, the term "volume" is used as a superset that includes "partition".[1][2][3]
In aUnix-like system, volumes are usually handled by theLogical Volume Manager or theEnterprise Volume Management System. Volumes are configured via themount command.
A volume other than the boot volume has a mount point somewhere within the filesystem, represented by a path. Logically, the directory tree stored on the volume is grafted in at the mount point. By convention, mount points are often placed in a directory called/mnt, though/media and other paths are also common. For example, if a CD-ROM drive containing a text file calledinfo.txt is mounted at/mnt/iso9660, the text file would be accessible at path/mnt/iso9660/info.txt. To use a path as a mount point, a directory must exist at that path.
Generally, a file in a volume can be moved to any other path within that volume by simply changing filesystemmetadata rather than copying file content. However, if a file is moved to a path that is on a different volume, then the file content is copied to the target and deleted from the source volume (which takes significantly longer to complete).
Often, a volume is identified within a system by either or both a user-assignable and morehuman-readable identifier and a more fixed and globally-unique identifier that may be less human-readable. Identifier attributes and use varies by computing context.
Use of volume serial numbers originated in the 1950s withmainframes. In theOS/360 line, it user-configurable, has a maximum length of six characters, is uppercase and must start with a letter. For example, "SYSRES" is often used for asystem residence volume. Operating systems may use the volume serial number as mount point name.[4]
Avolume label is a user-assignable identifier for a volume. In the FAT filesystem, a volume label is restricted to 11 characters (reflecting the8.3 restriction although not divided into name and extension) even whenlong file name is enabled. It is stored as an entry within a disk'sroot directory with a special volume-labelattribute bit set, and also copied to an 11-byte field within the ExtendedBIOS Parameter Block of the disk's boot sector. The label is stored asuppercase in FAT andVFAT, and cannot containspecial characters that are also disallowed for regular filenames. In the NTFS filesystem, the length of its volume label is restricted to 32 characters, and can include lowercase characters and evenUnicode.[5] InexFAT, the length is restricted to 11 characters, but can include lowercase and Unicode characters. A label can be changed inWindows Explorer via keyboard shortcutF2 while a volume is selected or via a volume's context (right-click) menu. Thelabel command supports changing the label from thecommand line.
Avolume serial number is aserial number assigned to a storage medium that is generally both unique and not editable by the user. It provides a consistent and reliable identifier for the volume. In particular, it allows for determining when a volume has been added to or removed from a system.Formatting a volume typically changes the serial number, but relabeling does not.[6] In the FAT andNTFS file systems, a volume serial number is used to determine if a volume is present in a drive or not, and to detect if it was exchanged with another one. This identification system was designed during their development ofOS/2.[7] It was introduced inMS-DOS 4.01 in 1988. The serial number is a 32-bit number determined by the real-time clock reading of the host computer when the volume is formatted. Previously, determination of whether a volume was swapped was done by reading thevolume label. However, even at that time the volume label was both optional and not required to be unique which resulted in incorrect detection of media changes.
Both thedir andvol commands report the label and serial number of a volume.
Partition A portion of the hard disk. In many cases, this is the entire hard disk space, but it needn't be. Volume A unit of disk space composed of one or more sections of one or more disks. Prior versions of Windows Server used volume only when referring to dynamic disks, but Windows Server 2008 uses it to mean partitions as well.
In Windows Server 2008 the distinction between volumes and partitions is somewhat murky. When using Disk Management, a regular partition on a basic disk is called a simple volume, even though technically a simple volume requires that the disk be a dynamic disk.
Windows Server 2008 simplifies the Disk Management user interface by using one set of dialog boxes and wizards for both partitions and volumes.