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ANSWERED May 2009:
In simple terms, a LUN is a slice or portion of a configured set of
disks that is presentable to a host and mounted as a volume within the
OS.
Internal architecture and front end connections aside, the primary
reason for purchasing a disk array is to provide data storage capacity
through presenting a number of physical disks, and making them available
and configurable to the IT administrator for the purposes of
distributing portions of that capacity (e.g. volumes) to specific
applications.
The disks in an array are usually configured into smaller sets (RAID
groups) to provide protection against failure. These RAID groups define
the manner in which the group of physical disks handle data when it is
written to the disks within that group (and as such, the way in which
that data is protected). However, a RAID group (being the underlying
structure of that group of physical disks), is not presentable to the
host. In order to do this, an administrator must create a logical unit (LUN) which can be presented to and mounted on a host operating system.
This LUN can either be configured as a small 'slice' of the total usable
capacity of the RAID group, or configured to use the total space
available within that RAID group, (dependant on the requirements of the
application that it wishes to use it). The LUN, when presented to the
host, shows as a mountable volume of the same capacity as the 'slice'
configured by the administrator, hiding any remaining capacity on that
RAID group. Any remaining capacity can then be 'sliced' into additional
LUNs as required.
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