Volume (computing)
Encyclopedia
In the context of computer operating system
s, volume is the term used to describe a single accessible storage area with a single file system
, typically (though not necessarily) resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on some media using a single instance of a filesystem. "Volume" can be used in place of the term "drive" where it is desirable to indicate that the entity in question is not a physical disk drive, but rather the corporate data stored by using a filesystem there. "Logical drive" and "volume" should be considered synonymous, however "volume" and "partition" are not synonymous. In Linux
systems, volumes are usually handled by the Logical Volume Manager
or the Enterprise Volume Management System
and manipulated using mount(8). The term is also used in NT
-based versions of Microsoft Windows
, where they are handled by the kernel and managed using the Disk Management MMC snap-in
.
might be accessible as a volume, even though it does not contain a partition, as floppy disks cannot be partitioned with most modern computer hardware. 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 ext3
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. Some times there is a one-to-one correspondence, but it is not guaranteed to be true.
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 aren't partitions.
system with two physical hard disks. The first hard disk has two partitions, the second has only one. The first partition of the first hard disk contains the operating system. Mount points have been left at defaults.
In this example,
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:
By default, Windows will assign drive letters to all drives, as follows:
Because of this convention, the operating system startup drive is most commonly called "C:". This is not always the case.
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 (Windows)
.
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.
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.
Like in Windows, to use a given path as a mount-point for another volume, an empty 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'.
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.
, allow multiple pseudonyms (known as "hard link
s") to be created for a single file within the same volume. Hard links allow a file to be referenced by two separate filenames, without its data being stored in two places on the disk (and thereby consuming twice as much space). Hard links cannot be created for files between volumes; this is comparable to moving the file in the library example above. To return to the library analogy, this is like filing two index cards for the same book: one could file the above book under both 'flora' and 'plants'. In general, deleting one hard link does not immediately effect other hard links, while deleting the final hard link for a file frees the disk space occupied by that file. However, modifying the data of the file referred to by one hard link will impact all other hard links as well. In the library, this is comparable to writing in the book. Hard links should not be confused with aliases (Mac OS), shortcuts
(Windows), or soft links (another type of link under Unix and variants), which can refer to files on another volume or no file at all. FAT
filesystems, such as FAT32, do not support hard or soft links as such, although the Windows operating system supports 'links', which are somewhat less capable.
filesystem, the volume label was traditionally restricted to 11 characters (reflecting the 8.3
restrictions, but not divided into name and extension fields) even when long file name was enabled, stored as an entry within a disk's root directory
with a special volume-label attribute
bit set, and also copied to an 11-byte field within the Extended BIOS Parameter Block
of the disk's boot sector.
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, volume is the term used to describe a single accessible storage area with a single file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
, typically (though not necessarily) resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on some media using a single instance of a filesystem. "Volume" can be used in place of the term "drive" where it is desirable to indicate that the entity in question is not a physical disk drive, but rather the corporate data stored by using a filesystem there. "Logical drive" and "volume" should be considered synonymous, however "volume" and "partition" are not synonymous. In Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
systems, volumes are usually handled by the Logical Volume Manager
Logical Volume Manager
Logical Volume Manager may refer to:*Logical Volume Manager *Logical Volume Manager...
or the Enterprise Volume Management System
Enterprise Volume Management System
Enterprise Volume Management System is a flexible, integrated volume management software used to manage storage systems under Linux.Its features include:* Handle EVMS, Linux LVM and LVM2 volumes* Handle many kinds of disk partitioning schemes...
and manipulated using mount(8). The term is also used in NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
-based versions of Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, where they are handled by the kernel and managed using the Disk Management MMC snap-in
Microsoft Management Console
Microsoft Management Console is a component of Windows 2000 and its successors that provides system administrators and advanced users an interface for configuring and monitoring the system.- Snap-ins and consoles :...
.
Differences between volume and partition
A volume is not the same thing as a partition. For example, a floppy diskFloppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
might be accessible as a volume, even though it does not contain a partition, as floppy disks cannot be partitioned with most modern computer hardware. 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
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
OS partitions, such as the ext3
Ext3
The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions, including Debian...
filesystem commonly used with Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
. 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. Some times there is a one-to-one correspondence, but it is not guaranteed to be true.
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 aren't partitions.
Example
This example concerns a Windows XPWindows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
system with two physical hard disks. The first hard disk has two partitions, the second has only one. The first partition of the first hard disk contains the operating system. Mount points 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".
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 NTFSNTFSNTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
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 volume must be formatted NTFS.
- A folder must exist at the root path. (As of Windows Vista, it can be any subfolder in a volume)
- That folder must be empty.
By default, Windows will assign drive letters to all drives, as follows:
- "A:" and "B:" to floppy disk drives, 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 convention, the operating system startup drive is most commonly called "C:". This is not always the case.
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 (Windows)
Control Panel (Windows)
The Control Panel is a part of the Microsoft Windows graphical user interface which allows users to view and manipulate basic system settings and controls via applets, such as adding hardware, adding and removing software, controlling user accounts, and changing accessibility options...
.
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-likeUnix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
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.
Like in Windows, to use a given path as a mount-point for another volume, an empty directory (sometimes called a folder) must exist there.
Unix-like operating systems use the mount
Mount (computing)
Mounting takes place before a computer can use any kind of storage device . The user or their operating system must make it accessible through the computer's file system. A user can access only files on mounted media.- Mount point :A mount point is a physical location in the partition used as a...
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'.
Speed of data management
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
Index card
An index card consists of heavy paper stock cut to a standard size, used for recording and storing small amounts of discrete data. It was invented by Carl Linnaeus, around 1760....
. However, to move the book to another library, adjusting index cards alone is insufficient. The entire book must be moved.
Special functions of advanced filesystems and volumes
Some filesystems, such as the Unix File System (ufs), Microsoft's NTFS filesystem, and ext3Ext3
The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions, including Debian...
, allow multiple pseudonyms (known as "hard link
Hard link
In computing, a hard link is a directory entry that associates a name with a file on a file system. . The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g...
s") to be created for a single file within the same volume. Hard links allow a file to be referenced by two separate filenames, without its data being stored in two places on the disk (and thereby consuming twice as much space). Hard links cannot be created for files between volumes; this is comparable to moving the file in the library example above. To return to the library analogy, this is like filing two index cards for the same book: one could file the above book under both 'flora' and 'plants'. In general, deleting one hard link does not immediately effect other hard links, while deleting the final hard link for a file frees the disk space occupied by that file. However, modifying the data of the file referred to by one hard link will impact all other hard links as well. In the library, this is comparable to writing in the book. Hard links should not be confused with aliases (Mac OS), shortcuts
Computer shortcut
A file shortcut in Microsoft Windows is a small file containing a target URI or GUID to an object, or the name of a target program file that the shortcut represents. The shortcut might additionally specify parameters to be passed to the target program when it is run. Each shortcut can have its own...
(Windows), or soft links (another type of link under Unix and variants), which can refer to files on another volume or no file at all. FAT
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
filesystems, such as FAT32, do not support hard or soft links as such, although the Windows operating system supports 'links', which are somewhat less capable.
Volume label
A volume label is the name given to a specific volume in a filesystem. In the FATFile Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
filesystem, the volume label was traditionally restricted to 11 characters (reflecting the 8.3
8.3
An 8.3 filename is a filename convention used by old versions of DOS, versions of Microsoft Windows prior to Windows 95, and Windows NT 3.51. It is also used in modern Microsoft operating systems as an alternate filename to the long filename for compatibility with legacy programs. The filename...
restrictions, but not divided into name and extension fields) even when long file name was enabled, stored as an entry within a disk's root directory
Root directory
In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...
with a special volume-label attribute
File attribute
A file attribute is metadata that describes or is associated with a computer file. For example, an operating system often keeps track of the date a file was created and last modified, as well as the file's size and extension . File permissions are also kept track of...
bit set, and also copied to an 11-byte field within the Extended BIOS Parameter Block
BIOS parameter block
In computing, the BIOS parameter block, often shortened to BPB, is a data structure in the Volume Boot Record describing the physical layout of a data storage volume. On partitioned devices, such as hard disks, the BPB describes the volume partition, whereas, on unpartitioned devices, such as...
of the disk's boot sector.
External links
- MSDN's article on Hard Links and Junctions