Non-RAID drive architectures
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||It has been suggested that this article be merged into RAID. (Discuss) Proposed since February 2013.|
The most widespread standard for configuring multiple hard drives is RAID (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 similarity to RAID, several tongue-in-cheek:
- JBOD: Just a bunch of disks; an array of drives, each of which can be accessed directly as an independent drive
- SPAN or BIG: A method of combining the free space on multiple drives to create a spanned volume. Such a concatenation is sometimes also called JBOD. The JBOD can be multiple individual logical disks or multiples in a spanned volume. A SPAN or BIG is generally a spanned volume only (contains mismatched types and sizes for non-raid).
- MAID: A system using hundreds to thousands of hard drives for nearline storage
JBOD (or just a bunch of disks) 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.
Concatenation (SPAN, BIG)
|This section's factual accuracy is disputed. (December 2012)|
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 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.
What makes a SPAN or BIG different from RAID configurations is the types of drives. RAID requires all drives be of like capacity and it is preferred they be of like model, a spanned volume does not have these requirements.
The initial release of Microsoft's Windows Home Server 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.
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 drives. The user will only ever see one copy of each file.
A massive array of idle drives (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. 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. MAID can supplement or replace tape libraries in hierarchical storage management.
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.
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