Volume (computing)

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In computer data storage, a volume or logical drive is a single accessible storage area with a single

.

Differences from partition

A volume is not the same thing as a partition. For example, a floppy disk might be accessible as a volume, even though it does not contain a partition, as floppy disks cannot be partitioned with most modern computer software. Also, an OS can recognize a partition without recognizing any volume associated with it, as when the OS cannot interpret the filesystem stored there. This situation occurs, for example, when Windows NT-based OSes encounter disks with non-Microsoft OS partitions, such as the ext4 filesystem commonly used with Linux. Another example occurs in the Intel world with the "Extended Partition". While these are partitions, they cannot contain a filesystem directly. Instead, "logical drives" (aka volumes) must be created within them. This is also the case with NetWare volumes residing inside of a single partition. In short, volumes exist at the logical OS level, and partitions exist at the physical, media specific level. Sometimes there is a one-to-one correspondence, but this is not guaranteed.

In Microsoft Windows Server 2008 and onward the term "volume" is used as a superset that includes "partition" as well.^[1][2][3]

It isn't uncommon to see a volume packed into a single file. Examples include ISO9660 disc images (CD/DVD images, commonly called "ISOs"), and installer volumes for Mac OS X (DMGs). As these volumes are files which reside within another volume, they certainly are not partitions.

Example

This example concerns a

have been left at defaults.

Physical disk	Partition	Filesystem	Drive letter
Hard Disk 1	Partition 1	NTFS	C:
	Partition 2	FAT32	D:
Hard Disk 2	Partition 1	FAT32	E:

In this example,

• "C:", "D:", and "E:" are volumes.
• Hard Disk 1 and Hard Disk 2 are physical disks.
• Any of these can be called a "drive".

Nomenclature

In

or the Diskpart command line tool.

Windows NT-based operating systems

It is important to note that Windows NT-based OSes do not have a single root directory. As a result, Windows will assign at least one path to each mounted volume, which will take one of two forms:

• A
  drive letter, in the form of a single letter followed by a colon
  , such as "F:"
• A mount-point on an NTFS volume having a drive letter, such as "C:\Music"

In these two examples, a file called "Track 1.mp3" stored in the root directory of the mounted volume could be referred to as "F:\Track 1.mp3" or "C:\Music\Track 1.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 will assign drive letters to all drives, as follows:

• "A:" and "B:" to floppy disk drives, whether present or not
• "C:" and subsequent letters, as needed, to:
  • Hard disks
  • Removable disks, including optical media (e.g. CDs and DVDs)

Because of this

), and as long as the OS has not been reinstalled on the computer.

On Windows XP, mount points may be managed through the Disk Management snap-in for the Microsoft Management Console. This can be most conveniently accessed through "Computer Management" in the "Administrative Tools" section of the Control Panel.

More than one drive letter can refer to a single volume, as when using the SUBST command.

Warning: removing drive letters or mount-points for a drive may break some programs, as some files may not be accessible under the known path. For example, if a program is installed at "D:\Program Files\Some Program", it may expect to find its data files at "D:\Program Files\Some Program\Data". If the logical disk previously called "D:" has its drive letter changed to "E:", "Some Program" won't be able to find its data at "D:\Program Files\Some Program\Data", since the drive letter "D:" no longer represents that volume.

Unix-like operating systems

In Unix-like operating systems, volumes other than the boot volume have a mount-point somewhere within the filesystem, represented by a path. Logically, the directory tree stored on the volume is grafted in at the mountpoint. By convention, mount-points will often be placed in a directory called '/mnt', though '/media' and other terms are sometimes used.

To use a given path as a mount-point for another volume, a directory (sometimes called a "folder") must exist there.

Unix-like operating systems use the mount command to manipulate mount points for volumes.

For example, if a CD-ROM drive containing a text file called 'info.txt' was mounted at '/mnt/iso9660', the text file would be accessible at '/mnt/iso9660/info.txt'.

Data management speed

Files within a volume can generally be moved to any other place within that volume by manipulating the filesystem, without moving the actual data. However, if a file is to be moved outside the volume, the data itself must be relocated, which is a much more expensive operation.

In order to better visualize this concept, one might consider the example of a large library. If a non-fiction work is originally classified as having the subject "plants", but then has to be moved to the subject "flora", one does not need to refile the book, whose position on the shelf would be static, but rather, one needs only to replace the index card. However, to move the book to another library, adjusting index cards alone is insufficient. The entire book must be moved.

Labels and serial numbers

A volume label is the name given to a specific volume in a filesystem. In the

.

In contrast to the label, the volume serial number is generally unique and is not normally changed by the user, and thus acts as a more consistent and reliable identifier of when a volume has been changed (as when a disk is removed and another inserted).

A volume serial number is a

tape volume

. In FAT and NTFS file systems, a volume serial number is a feature used to determine if a disk is present in a drive or not, and to detect if it was exchanged with another one. This identification system was created by in 1988.

The volume serial number is a 32-bit number determined by the date and time on the real-time clock on the current computer at the time of a disk's formatting. Previously, determination by the OS of whether a disk was swapped was done by reading the drive's volume label. However, even at that time the volume label was not required to be unique and was optional. Therefore, many users had not given disks any meaningful name and the old method failed.

The

to display the current label and serial number of a volume.

References

External links

• MSDN's article on Hard Links and Junctions