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The most widespread standard for configuring multiple
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...
s is
RAID Raid, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...
(Redundant Array of Inexpensive/Independent Disks), which comes in a number of standard configurations and non-standard configurations. Non-RAID drive architectures also exist, and are referred to by acronyms with
tongue-in-cheek The idiom tongue-in-cheek refers to a humorous or sarcastic statement expressed in a serious manner. History The phrase originally expressed contempt, but by 1842 had acquired its modern meaning. Early users of the phrase include Sir Walter Scot ...
similarity to RAID: * JBOD (derived from "just a bunch of disks"): described multiple hard disk drives operated as individual independent hard disk drives. * SPAN or BIG: A method of combining the free space on multiple hard disk drives from "JBoD" to create a spanned volume. Such a concatenation is sometimes also called BIG/SPAN. A SPAN or BIG is generally a spanned volume only, as it often contains mismatched types and sizes of hard disk drives. * MAID (derived from "massive array of idle drives"): an architecture using hundreds to thousands of hard disk drives for providing
nearline storage Nearline storage (a portmanteau of " near" and "online storage") is a term used in computer science to describe an intermediate type of data storage that represents a compromise between online storage (supporting frequent, very rapid access to dat ...
of
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
, primarily designed for "Write Once, Read Occasionally" (WORO) applications, in which increased storage density and decreased cost are traded for increased latency and decreased redundancy.


JBOD

JBOD (abbreviated from "Just a Bunch Of Disks"/"Just a Bunch Of Drives") is an architecture using multiple hard drives exposed as individual devices. Hard drives may be treated independently or may be combined into one or more logical volumes using a volume manager like LVM or
mdadm mdadm is a Linux utility used to manage and monitor software RAID devices. It is used in modern Linux distributions in place of older software RAID utilities such as raidtools2 or raidtools. mdadm is free software originally maintained by, ...
, or a device-spanning filesystem like
btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
; such volumes are usually called "spanned" or "linear , SPAN , BIG". A spanned volume provides no redundancy, so failure of a single hard drive amounts to failure of the whole logical volume. Redundancy for resilience and/or bandwidth improvement may be provided, in software, at a higher level.


Concatenation (SPAN, BIG)

Concatenation or spanning of drives is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single logical disk. It provides no data redundancy. Drives are merely
concatenated In formal language theory and computer programming, string concatenation is the operation of joining character strings end-to-end. For example, the concatenation of "snow" and "ball" is "snowball". In certain formalisations of concatenat ...
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). In the adjacent diagram, 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. What makes a SPAN or BIG different from RAID configurations is the possibility for the selection of drives. While RAID usually requires all drives to be of similar capacity and it is preferred that the same or similar drive models are used for performance reasons, a spanned volume does not have such requirements.


Implementations

The initial release of Microsoft's
Windows Home Server Windows Home Server (code-named Quattro) is a home server operating system from Microsoft. It was announced on 7 January 2007 at the Consumer Electronics Show by Bill Gates, released to manufacturing on 16 July 2007 and officially released o ...
employs drive extender technology, whereby an array of independent drives 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 drives, 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. The
btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
filesystem can span multiple devices of different sizes, including RAID 0/1/10 configurations, storing 1 to 4 redundant copies of both data and metadata. (A flawed RAID 5/6 also exists, but can result in data loss.) For RAID 1, the devices must have complementary sizes. For example, a filesystem spanning two 500 GB devices and one 1 TB device could provide RAID1 for all data, while a filesystem spanning a 1 TB device and a single 500 GB device could only provide RAID1 for 500 GB of data. The
ZFS ZFS (previously: Zettabyte File System) is a file system with volume management capabilities. It began as part of the Sun Microsystems Solaris operating system in 2001. Large parts of Solaris – including ZFS – were published under an ope ...
filesystem can likewise pool multiple devices of different sizes and implement RAID, though it is less flexible, requiring the creation of virtual devices of fixed size on each device before pooling. In enterprise environments, enclosures are used to expand a server's data storage by using JBOD devices. This is often a convenient way to scale-up storage when needed by daisy-chaining additional disk shelves.


MAID

MAID (abbreviated from "massive array of idle drives") is an architecture using hundreds to thousands of hard drives for providing
nearline storage Nearline storage (a portmanteau of " near" and "online storage") is a term used in computer science to describe an intermediate type of data storage that represents a compromise between online storage (supporting frequent, very rapid access to dat ...
of data. MAID is designed for "Write Once, Read Occasionally" (WORO) applications. Compared to RAID technology, MAID has increased storage density, and decreased cost, electrical power, and cooling requirements. However, these advantages are at the cost of much increased latency, significantly lower
throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ov ...
, and decreased redundancy. Drives designed for multiple spin-up/down cycles (e.g. laptop drives) are significantly more expensive. Latency may be as high as tens of seconds.Cook, Rick (2004-07-12)
"Backup budgets have it MAID with cheap disk" Retrieved on 2008-07-15
/ref> MAID can supplement or replace
tape libraries In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device that contains one or more tape drives, a number of slots to hold tape cartridges, a barcode reader to identify tape cartridges ...
in
hierarchical storage management Hierarchical storage management (HSM), also known as Tiered storage, is a data storage and Data management technique that automatically moves data between high-cost and low-cost storage media. HSM systems exist because high-speed storage devices, ...
. To allow a more gradual tradeoff between access time and power savings, some MAIDs such as Nexsan's AutoMAID incorporate drives capable of spinning down to a lower speed. Large scale disk storage systems based on MAID architectures allow dense packaging of drives and are designed to have only 25% of disks spinning at any one time.


See also

*
File-based replication Replication in computing involves sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility. Terminology Replication in comp ...
*
Nested RAID levels Nested RAID levels, also known as hybrid RAID, combine two or more of the standard RAID levels (where "RAID" stands for "redundant array of independent disks") to gain performance, additional redundancy or both, as a result of combining properties ...
*
Non-standard RAID levels Although all RAID implementations differ from the specification to some extent, some companies and open-source projects have developed non-standard RAID implementations that differ substantially from the standard. Additionally, there are non-RAI ...
*
Standard RAID levels In computer storage, the standard RAID levels comprise a basic set of RAID ("redundant array of independent disks" or "redundant array of inexpensive disks") configurations that employ the techniques of striping, mirroring, or parity to create lar ...


Explanatory notes


References

{{Reflist, 30em


External links


PCGuide – "Just A Bunch Of Disks" (JBOD)
RAID