Volume (computing)

Storage can be designed and configured in many different and complex ways yet all include the volume concept. A general-purpose mass-storage device is typically divided into partitions with each partition used as a volume with its own file system. A more specialized mass-storage device may be configured for redundancy where a volume spans multiple storage media or physical drives. A smaller storage such as a memory card or floppy disk might have only one partition or no partition at all (no partition table) such that the volume is logically indistinguishable from the drive. A simple volume describes the most basic configuration: a volume on one storage medium with no redundancy or striping.

The following table describes an example storage configuration for a Windows 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 letters C:, D:, and E: each identify a volume, a.k.a. logical drive.

An operating 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 the ext4 filesystem commonly used with Linux.

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 are not partitions.

The concept of volume applies to any type of storage medium. But, for historical reasons, the term disk is often used even for non-disk media. For example, the Windows Disk Management utility supports any type of medium.

Operating system support

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.

Microsoft systems

In Windows, volumes are handled by the kernel. Volumes and mount points and configured by the user via the Disk Management utility or the diskpart command.

Unlike in a Unix-like system, Microsoft systems do not have a single root directory. The system assigns at least one path to each mounted volume – either a drive letter (shown as a letter followed by a colon) such as F: or a mount point on an NTFS volume having a drive letter, such as C:\Music. In this case, the file Track1.mp3 stored in the root directory of the mounted volume could be referred to via paths F:\Track1.mp3 or C:\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:

• The mounted-to volume must be formatted NTFS
• A directory must exist at the root path; as of Windows Vista, it can be any subdirectory in a volume
• That directory must be empty

By default, Windows assigns drive letters to all drives. It reserves A: and B: for floppy disk drives, whether present or not. It uses C: and subsequent letters for all other drives (e.g. SDD, HDD, CD, DVD). The operating system startup drive is most commonly assigned C:, however this is not always the case. Since personal computers now no longer include floppies, and optical disc and other removable drives typically still start at D:, letters A and B are available for assignment by a user with administrative 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 its volume serial number).

More than one drive letter can refer to a single volume. This can be configured via the SUBST 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 directory D:\Data and then drive letter D: is unassigned, the program no longer can access its data since D:\Data is no longer a valid path.

In Windows Server 2008 and onward, the term "volume" is used as a superset that includes "partition".

Unix-like systems

In a Unix-like system, volumes are usually handled by the Logical Volume Manager or the Enterprise Volume Management System. Volumes are configured via the mount 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 called info.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.