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.
- 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, meaning "Just a Bunch Of Drives", is used to refer to one distinct concept:
- all disks are independently addressed, with no collective properties. Each physical disk, with all the logical partitions each may contain, is mapped to a different logical volume: just a bunch of disks.
The concept of concatenation, where all the physical disks are concatenated and presented as a single disk, is NOT a JBOD, but is properly called BIG or SPAN. The usage of "JBOD" and BIG or SPAN are frequently confused. This can create confusion and frustration, given the significantly different logical arrangement of the various types. Concatenation, referred to by such unambiguous terms as SPAN or BIG, requires software to bond/append drives together, where JBOD does not.
Concatenation (SPAN, BIG)
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. 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".
The Mac OS X 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.
A "massive array of idle disks" (more commonly known as a MAID) provides higher density and more environmentally efficient drive arrays for near-line data storage.