Non-RAID drive architectures

The most widespread standard for configuring multiple hard disk 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 (abbreviated from "just a bunch of disks/drives") is an architecture using multiple hard drives exposed as individual devices. Hard drives may be treated independently or may be combined into a one or more logical volumes using a volume manager like LVM or mdadm; 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 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 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.

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.

...
Wikipedia