Non-RAID drive architectures
Encyclopedia
The most widespread standard for configuring multiple hard drives is RAID
(Redundant Array of Independent Disks), which comes in a number of standard configurations
and non-standard configurations
. Non-RAID drive architectures also exist, and are often referred to, similarly to RAID, by standard acronyms, several tongue-in-cheek.
Note that neither concept implies data redundancy. The usage is contentious; in careful usage, JBOD refers to the first (independent disks), as there is no other term to refer to this, while concatenation is referred to by unambiguous terms such as SPAN or BIG
.
For example, in the context of ZFS
, JBOD refers to independent drives.
The host controller of a JBOD setup may also impact how it can be used. A USB or FireWire host connection will address multiple physical drives, but an eSATA connection requires a SATA
host controller with Port Multiplier support, or else it will only recognize the first physical disk.
Concatenation or spanning
of disks is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single virtual disk. It provides no data redundancy. As the name implies, disks are merely concatenated
together, end to beginning, so they appear to be a single large disk. It may be referred to as SPAN or BIG (meaning just the words "span" or "big", not as acronyms).
Concatenation may be thought of as the inverse of partitioning
. Whereas partitioning takes one physical drive and creates two or more logical drives, concatenation uses two or more physical drives to create one logical drive.
In that it consists of an array of independent disks, it can be thought of as a distant relative of RAID. Concatenation is sometimes used to turn several odd-sized drives into one larger useful drive, which cannot be done with RAID 0. For example, one could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the individual drives separately.
In the diagram to the right, data are concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks.
Many Linux
distributions use the terms "linear mode" or "append mode".
10.4 implementation – called a "Concatenated Disk Set" – does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above.
Concatenation is one of the uses of the Logical Volume Manager
in Linux, which can be used to create virtual drives spanning multiple physical drives and/or partitions.
The initial release of Microsoft's Windows Home Server
employs drive extender technology, whereby an array of independent disks are combined by the OS to form a single pool of available storage. This storage is presented to the user as a single set of network shares. Drive extender technology expands on the normal features of concatenation by providing data redundancy through software – a shared folder can be marked for duplication, which signals to the OS that a copy of the data should be kept on multiple physical disks, whilst the user will only ever see a single instance of their data. This feature was removed from Windows Home Server in its subsequent major release.
Greyhole implements what it calls a "storage pool". This pool is created by presenting to the user, through Samba
shares, a logical drive that is as large as the sum of all physical drives that are part of the pool. Greyhole also provides data redundancy through software - the user can configure, per share, the number of file copies that Greyhole is to maintain. Greyhole will then ensure that for each file in such shares, the correct number of extra copies are created and maintained on multiple physical disks. The user will only ever see one copy of each file.
" (more commonly known as a MAID) provides higher density and more environmentally efficient drive arrays for near-line
data
storage.
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
(Redundant Array of Independent Disks), which comes in a number of standard configurations
Standard RAID levels
The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring, or parity.The standard RAID levels can be modified for other benefits ; there are also non-standard RAID levels, and non-RAID drive architectures, which may be offered as alternatives to RAID architectures...
and non-standard configurations
Non-standard RAID levels
Although all RAID implementations differ from the specification to some extent, some companies have developed non-standard RAID implementations that differ substantially from the standard...
. Non-RAID drive architectures also exist, and are often referred to, similarly to RAID, by standard acronyms, several tongue-in-cheek.
- a : A single, large, expensive drive.
- JBOD : Just a bunch of disks; an array of drives, each of which is accessed directly as an independent drive.
- a SPAN, a BIG: a simple concatenation of multiple drives. Such a concatenation is sometimes also called JBOD, but this usage is proscribed in careful use due to ambiguity with the alternative meaning just cited.
JBOD
JBOD, meaning "Just a Bunch Of Drives", is used to refer to two distinct concepts:- all disks being independently addressed, with no collective properties – each physical disk, with all the logical partitions each may contain, being mapped to a different logical volume: just a bunch of disks.
- concatenation, where all the physical disks are concatenated and presented as a single disk.
Note that neither concept implies data redundancy. The usage is contentious; in careful usage, JBOD refers to the first (independent disks), as there is no other term to refer to this, while concatenation is referred to by unambiguous terms such as SPAN or BIG
Spanned volume
A spanned volume is a formatted partition which data is stored on more than one hard disk, yet appears as one volume.Spanned volumes are a non-RAID drive architecture, and may be implemented in hardware or software; they may be referred to as Concatenation, SPAN, BIG, or JBOD, though this latter is...
.
For example, in the context of 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,...
, JBOD refers to independent drives.
The host controller of a JBOD setup may also impact how it can be used. A USB or FireWire host connection will address multiple physical drives, but an eSATA connection requires a SATA
Sata
Sata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak...
host controller with Port Multiplier support, or else it will only recognize the first physical disk.
Concatenation (SPAN, BIG)
Spanned volume
A spanned volume is a formatted partition which data is stored on more than one hard disk, yet appears as one volume.Spanned volumes are a non-RAID drive architecture, and may be implemented in hardware or software; they may be referred to as Concatenation, SPAN, BIG, or JBOD, though this latter is...
of disks is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single virtual disk. It provides no data redundancy. As the name implies, disks are merely concatenated
Concatenation
In computer programming, string concatenation is the operation of joining two character strings end-to-end. For example, the strings "snow" and "ball" may be concatenated to give "snowball"...
together, end to beginning, so they appear to be a single large disk. It may be referred to as SPAN or BIG (meaning just the words "span" or "big", not as acronyms).
Concatenation may be thought of as the inverse of partitioning
Disk partitioning
Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...
. Whereas partitioning takes one physical drive and creates two or more logical drives, concatenation uses two or more physical drives to create one logical drive.
In that it consists of an array of independent disks, it can be thought of as a distant relative of RAID. Concatenation is sometimes used to turn several odd-sized drives into one larger useful drive, which cannot be done with RAID 0. For example, one could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the individual drives separately.
In the diagram to the right, data are concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks.
Many 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...
distributions use the terms "linear mode" or "append mode".
Implementations
The Mac OS XMac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
10.4 implementation – called a "Concatenated Disk Set" – does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above.
Concatenation is one of the uses of the 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...
in Linux, which can be used to create virtual drives spanning multiple physical drives and/or partitions.
The initial release of Microsoft's Windows Home Server
Windows Home Server
Windows Home Server, code-named Quattro, is a home server operating system from Microsoft. Announced on 7 January 2007, at the Consumer Electronics Show by Bill Gates, Windows Home Server is intended to be a solution for homes with multiple connected PCs to offer file sharing, automated backups,...
employs drive extender technology, whereby an array of independent disks are combined by the OS to form a single pool of available storage. This storage is presented to the user as a single set of network shares. Drive extender technology expands on the normal features of concatenation by providing data redundancy through software – a shared folder can be marked for duplication, which signals to the OS that a copy of the data should be kept on multiple physical disks, whilst the user will only ever see a single instance of their data. This feature was removed from Windows Home Server in its subsequent major release.
Greyhole implements what it calls a "storage pool". This pool is created by presenting to the user, through Samba
Samba (software)
Samba is a free software re-implementation, originally developed by Andrew Tridgell, of the SMB/CIFS networking protocol. As of version 3, Samba provides file and print services for various Microsoft Windows clients and can integrate with a Windows Server domain, either as a Primary Domain...
shares, a logical drive that is as large as the sum of all physical drives that are part of the pool. Greyhole also provides data redundancy through software - the user can configure, per share, the number of file copies that Greyhole is to maintain. Greyhole will then ensure that for each file in such shares, the correct number of extra copies are created and maintained on multiple physical disks. The user will only ever see one copy of each file.
MAID
A "massive array of idle disksMassive array of idle disks
In computing, a massive array of idle disks is a system using hundreds to thousands of hard drives for nearline storage of data. MAID is designed for 'Write Once, Read Occasionally' applications. In a MAID each drive is only spun up on demand as needed to access the data stored on that drive...
" (more commonly known as a MAID) provides higher density and more environmentally efficient drive arrays for near-line
Nearline storage
Nearline storage is a term used in computer science to describe an intermediate type of data storage that represents a compromise between online storage and offline storage/archiving...
data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
storage.
See also
- RAIDRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
- Nested RAID levelsNested RAID levelsLevels of nested RAID, also known as hybrid RAID, combine two or more of the standard levels of RAID to gain performance, additional redundancy, or both.-Nesting:...
- Non-standard RAID levelsNon-standard RAID levelsAlthough all RAID implementations differ from the specification to some extent, some companies have developed non-standard RAID implementations that differ substantially from the standard...
- Standard RAID levelsStandard RAID levelsThe standard RAID levels are a basic set of RAID configurations and employ striping, mirroring, or parity.The standard RAID levels can be modified for other benefits ; there are also non-standard RAID levels, and non-RAID drive architectures, which may be offered as alternatives to RAID architectures...