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Non-RAID drive architectures

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See also: RAID

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 referred to by acronyms with similarity to RAID, several tongue-in-cheek:

  • JBOD: Just a bunch of disks; an array of drives, each of which is accessed directly as an independent drive.
  • SPAN or 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.[citation needed]
  • MAID: a system using hundreds to thousands of hard drives for nearline storage

JBOD

JBOD (Just a bunch of disks or Just a bunch of drives) is when a number of drives are used, but are not in a RAID configuration. The drives may be handled as separate logical volumes, or they may be combined into a single logical volume using a system like LVM.[1]

Concatenation (SPAN, BIG)

Main article: Spanned volume
Diagram of a SPAN/BIG ("JBOD") setup.

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. 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).

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.

Implementations

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.[2] This feature was removed from Windows Home Server in its subsequent major release.[3]

Greyhole, a disk-pooling application, 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.[4]

MAID

A massive array of idle disks (more commonly known as a MAID) is a system using hundreds to thousands of hard drives for nearline storage of data. MAID is designed for 'Write Once, Read Occasionally' (WORO) applications.

Compared to RAID technology a 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, and decreased redundancy. Low drive utilization rates may actually reduce reliability in consumer-oriented large PATA and SATA drives.[5] Drives designed for multiple spin-up/down cycles (e.g. laptop drives) are significantly more expensive.[6] Latency may be as high as tens of seconds.[7] MAID can supplement or replace tape libraries in hierarchical storage management.

To allow a more gradual tradeoff between access time and power savings, some MAIDs such as Nexsan's AutoMAID incorporate disks capable of spinning down to a lower speed.[8]

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.[7]

See also

References

  1. ^ http://searchstorage.techtarget.com/definition/JBOD. {{cite web}}: Missing or empty |title= (help)
  2. ^ "Windows Home Server Drive Extender Technical Brief". Microsoft.com. Retrieved 2009-03-12.
  3. ^ "Windows Home Server code name "Vail"– Update".
  4. ^ "Greyhole project on Google Code". Guillaume Boudreau.
  5. ^ Harris, Rick (2007-02-19). "Failure Trends in a Large Disk Drive Population", Google, Retrieved on 2012-08-28
  6. ^ SGI (2012). "Enterprise MAID Quick Reference Guide" (PDF). Retrieved 28 August 2012.
  7. ^ a b Cook, Rick (2004-07-12). "Backup budgets have it MAID with cheap disk" Retrieved on 2008-07-15
  8. ^ Nexsan (2011). "AutoMAID Energy Saving Technology". Retrieved 7 April 2011.
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