Logical volume management
Encyclopedia
In computer storage
, logical volume management or LVM provides a method of allocating space on mass-storage
devices that is more flexible than conventional partitioning schemes. In particular, a volume manager can concatenate, stripe
together or otherwise combine partitions into larger virtual ones that administrators can re-size or move, potentially without interrupting system use.
Volume management represents just one of many forms of storage virtualization
; its implementation takes place in a layer in the device-driver
stack of an OS
(as opposed to within storage devices or in a network).
s, hard disk partitions, or Logical Unit Numbers (LUNs) of an external storage device. Volume management treats PVs as sequences of chunks called physical extents (PEs). Some volume managers (such as that in HP-UX and Linux) have PEs of a uniform size; others (such as that in Veritas
) have variably-sized PEs that can be split and merged at will.
Normally, PEs simply map one-to-one to logical extents (LEs). With mirroring, multiple PEs map to each LE. These PEs are drawn from a physical volume group (PVG), a set of same-sized PVs which act similarly to hard disks in a RAID1 array. PVGs are usually laid out so that they reside on different disks
and/or data buses
for maximum redundancy.
The system pools LEs into a volume group (VG). The pooled LEs can then be concatenated together into virtual disk partitions called logical volumes or LVs. Systems can use LVs as raw block devices just like disk partitions: creating mountable file system
s on them, or using them as swap
storage.
Striped LVs allocate each successive LE from a different PV; depending on the size of the LE, this can improve performance on large sequential reads by bringing to bear the combined read-throughput of multiple PVs.
Administrators can grow LVs (by concatenating more LEs) or shrink them (by returning LEs to the pool). The concatenated LEs do not have to be contiguous. This allows LVs to grow without having to move already-allocated LEs. Some volume managers allow the re-sizing of LVs in either direction while online. Changing the size of the LV does not necessarily change the size of a filesystem
on it; it merely changes the size of its containing space. A file system that can be resized online is recommended in that it allows the system to adjust its storage on-the-fly without interrupting applications.
PVs and LVs cannot be shared between or span different VGs (although some volume managers may allow moving them at will between VGs on the same host). This allows administrators conveniently to bring VGs online, to take them offline or to move them between host systems as a single administrative unit.
VGs can grow their storage pool by absorbing new PVs or shrink by retracting from PVs. This may involve moving already-allocated LEs out of the PV. Most volume managers can perform this movement online; if the underlying hardware is hot-pluggable this allows engineers to upgrade or replace storage without system downtime.
by applying copy-on-write
to each LE. In this scheme, the volume manager will copy the LE to a copy-on-write table just before it is written to. This preserves an old version of the LV—the snapshot—which systems can later reconstruct by overlaying the copy-on-write table atop the current LV. Read-write snapshots are branching snapshots because they implicitly allow diverging versions of an LV.
Snapshots can be useful for backing up self-consistent versions of volatile data like table files from a busy database, or for rolling back large changes (such as an operating system upgrade) in a single operation. Some Linux
-based Live CD
systems also use snapshots to simulate read-write access on a read-only compact disc
.
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
, logical volume management or LVM provides a method of allocating space on mass-storage
Mass storage
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. Devices and/or systems that have been described as mass storage include tape libraries, RAID systems, hard disk drives, magnetic tape drives, optical disc drives, magneto-optical...
devices that is more flexible than conventional partitioning schemes. In particular, a volume manager can concatenate, stripe
Data striping
In computer data storage, data striping is the technique of segmenting logically sequential data, such as a file, in a way that accesses of sequential segments are made to different physical storage devices. Striping is useful when a processing device requests access to data more quickly than a...
together or otherwise combine partitions into larger virtual ones that administrators can re-size or move, potentially without interrupting system use.
Volume management represents just one of many forms of storage virtualization
Storage Virtualization
Storage virtualization or storage virtualisation is a concept and term used within computer science. Specifically, storage systems may use virtualization concepts as a tool to enable better functionality and more advanced features within the storage system.Broadly speaking, a 'storage system' is...
; its implementation takes place in a layer in the device-driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
stack of an OS
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...
(as opposed to within storage devices or in a network).
Design
Most volume-manager implementations share the same basic design. They start with physical volumes (PVs), which can be either hard diskHard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
s, hard disk partitions, or Logical Unit Numbers (LUNs) of an external storage device. Volume management treats PVs as sequences of chunks called physical extents (PEs). Some volume managers (such as that in HP-UX and Linux) have PEs of a uniform size; others (such as that in Veritas
Veritas Volume Manager
The Veritas Volume Manager, VVM or VxVM is a proprietary logical volume manager from Veritas . It is available for Windows, AIX, Solaris, Linux, and HP-UX. A modified version is bundled with HP-UX as its built-in volume manager...
) have variably-sized PEs that can be split and merged at will.
Normally, PEs simply map one-to-one to logical extents (LEs). With mirroring, multiple PEs map to each LE. These PEs are drawn from a physical volume group (PVG), a set of same-sized PVs which act similarly to hard disks in a RAID1 array. PVGs are usually laid out so that they reside on different disks
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
and/or data buses
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
for maximum redundancy.
The system pools LEs into a volume group (VG). The pooled LEs can then be concatenated together into virtual disk partitions called logical volumes or LVs. Systems can use LVs as raw block devices just like disk partitions: creating mountable 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...
s on them, or using them as swap
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
storage.
Striped LVs allocate each successive LE from a different PV; depending on the size of the LE, this can improve performance on large sequential reads by bringing to bear the combined read-throughput of multiple PVs.
Administrators can grow LVs (by concatenating more LEs) or shrink them (by returning LEs to the pool). The concatenated LEs do not have to be contiguous. This allows LVs to grow without having to move already-allocated LEs. Some volume managers allow the re-sizing of LVs in either direction while online. Changing the size of the LV does not necessarily change the size of a filesystem
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...
on it; it merely changes the size of its containing space. A file system that can be resized online is recommended in that it allows the system to adjust its storage on-the-fly without interrupting applications.
PVs and LVs cannot be shared between or span different VGs (although some volume managers may allow moving them at will between VGs on the same host). This allows administrators conveniently to bring VGs online, to take them offline or to move them between host systems as a single administrative unit.
VGs can grow their storage pool by absorbing new PVs or shrink by retracting from PVs. This may involve moving already-allocated LEs out of the PV. Most volume managers can perform this movement online; if the underlying hardware is hot-pluggable this allows engineers to upgrade or replace storage without system downtime.
Snapshots
Some volume managers also implement snapshotsSnapshot (computer storage)
In computer systems, a snapshot is the state of a system at a particular point in time. The term was coined as an analogy to that in photography. It can refer to an actual copy of the state of a system or to a capability provided by certain systems....
by applying copy-on-write
Copy-on-write
Copy-on-write is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, they can all be given pointers to the same resource...
to each LE. In this scheme, the volume manager will copy the LE to a copy-on-write table just before it is written to. This preserves an old version of the LV—the snapshot—which systems can later reconstruct by overlaying the copy-on-write table atop the current LV. Read-write snapshots are branching snapshots because they implicitly allow diverging versions of an LV.
Snapshots can be useful for backing up self-consistent versions of volatile data like table files from a busy database, or for rolling back large changes (such as an operating system upgrade) in a single operation. Some 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...
-based Live CD
Live CD
A live CD, live DVD, or live disc is a CD or DVD containing a bootable computer operating system. Live CDs are unique in that they have the ability to run a complete, modern operating system on a computer lacking mutable secondary storage, such as a hard disk drive...
systems also use snapshots to simulate read-write access on a read-only compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
.
Implementations
| Vendor | Introduced in | Volume manager | Allocate anywhere | Snapshots | RAID 0 | RAID 1 | RAID 5 | RAID 10 | Notes |
|---|---|---|---|---|---|---|---|---|---|
| IBM IBM International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas... |
AIX AIX operating system AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms... 3.0 (1989) |
Logical Volume Manager | |||||||
| Hewlett-Packard Hewlett-Packard Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including... |
HP-UX HP-UX HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984... 9.0 |
HP Logical Volume Manager | |||||||
| FreeBSD FreeBSD FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant... |
Vinum Volume Manager Vinum volume manager Vinum, is a logical volume manager, also called Software RAID, allowing implementations of the RAID-0, RAID-1 and RAID-5 models, both individually and in combination. Vinum is part of the base distribution of the FreeBSD operating system. Versions exist for NetBSD, OpenBSD and DragonFly BSD. Vinum... |
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| NetBSD NetBSD NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,... |
Logical Volume Manager Logical Volume Manager Logical Volume Manager may refer to:*Logical Volume Manager *Logical Volume Manager... |
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| 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... 2.2 |
Logical Volume Manager Logical Volume Manager (Linux) LVM is a logical volume manager for the Linux kernel; it manages disk drives and similar mass-storage devices, in particular large ones. The term "volume" refers to a disk drive or partition thereof... |
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| 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... 2.4 |
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... |
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| 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... 2.6 |
Logical Volume Manager Logical Volume Manager (Linux) LVM is a logical volume manager for the Linux kernel; it manages disk drives and similar mass-storage devices, in particular large ones. The term "volume" refers to a disk drive or partition thereof... |
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| Silicon Graphics Silicon Graphics Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark... |
IRIX IRIX IRIX is a computer operating system developed by Silicon Graphics, Inc. to run natively on their 32- and 64-bit MIPS architecture workstations and servers. It was based on UNIX System V with BSD extensions. IRIX was the first operating system to include the XFS file system.The last major version... or 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... |
XVM Volume Manager | |||||||
| Sun Microsystems Sun Microsystems Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982... |
SunOS SunOS SunOS is a version of the Unix operating system developed by Sun Microsystems for their workstation and server computer systems. The SunOS name is usually only used to refer to versions 1.0 to 4.1.4 of SunOS... |
Solaris Volume Manager Solaris Volume Manager Solaris Volume Manager is a software package for creating, modifying and controlling RAID-0 volumes, RAID-1 volumes, RAID 0+1 volumes, RAID 1+0 volumes, RAID-5 volumes, and soft partitions.Version 1.0 of Online: DiskSuite was released as an add-on product for SunOS in... (was Solstice DiskSuite). |
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| Sun Microsystems Sun Microsystems Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982... |
Solaris 10 | ZFS ZFS In computing, ZFS is a combined file system and logical volume manager designed by Sun Microsystems. The features of ZFS include data integrity verification against data corruption modes , support for high storage capacities, integration of the concepts of filesystem and volume management,... |
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| Veritas VERITAS Software Veritas Software Corp. was an international software company that was founded in 1983 as Tolerant Systems, renamed Veritas Software Corp. in 1989, and merged with Symantec in 2005. It was headquartered in Mountain View, California... |
Cross-OS | Veritas Volume Manager Veritas Volume Manager The Veritas Volume Manager, VVM or VxVM is a proprietary logical volume manager from Veritas . It is available for Windows, AIX, Solaris, Linux, and HP-UX. A modified version is bundled with HP-UX as its built-in volume manager... (VxVM) |
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| 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... |
Windows 2000 Windows 2000 Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the... and later NT-based operating systems |
Logical Disk Manager Logical Disk Manager The Logical Disk Manager is an implementation of a logical volume manager for Microsoft Windows NT, developed by Microsoft and Veritas Software. It was introduced with the Windows 2000 operating system, and is supported in Windows XP, Windows Server 2003, Windows Vista and Windows 7... |
Disadvantages
- The levels of indirection that volume managers introduce can complicate the boot process and make disaster recoveryDisaster recoveryDisaster recovery is the process, policies and procedures related to preparing for recovery or continuation of technology infrastructure critical to an organization after a natural or human-induced disaster. Disaster recovery is a subset of business continuity...
difficult, especially when the base operating-system and other essential tools are themselves on an LV. - Logical volumes can suffer from external fragmentation when the underlying storage devices do not allocate their PEs contiguously. This can reduce I/O performance on slow-seeking media (such as magnetic disks), which have to seek over the gaps between extents during large sequential reads or writes. Volume managers which use fixed-size PEs, however, typically make PEs relatively large (a default of 4MB on the Linux LVM, for example) in order to amortizeAmortizationAmortization is the process of decreasing, or accounting for, an amount over a period. The word comes from Middle English amortisen to kill, alienate in mortmain, from Anglo-French amorteser, alteration of amortir, from Vulgar Latin admortire to kill, from Latin ad- + mort-, mors death.When used...
the cost of these seeks.