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March 14, 2012

We explore logical unit numbers (LUNs). What is the definition of a LUN, and how do LUNs relate to volumes.

( Click Me and Read from Page 8 for LUNS usage )

OS’s must speak with physical disk in a language they understand

The bedrock of storage is the disk drive, but an operating system cannot access disk drives without mediation via logical addressing that translates the physical characteristics of the disk – platters, heads, tracks and sectors – into a language the operating system can understand.

LUNs are ‘soft’ partitions

There is no 1:1 relationship between physical disk drives and LUNs. When provisioning storage, the administrator uses management software to create LUNs. They can create, for example, more than one LUN from one physical drive, which would then appear as two or more discrete drives to the user. Or they may create a number of LUNs that span several separate disks that form a RAID array; but, again, these will appear as discrete drives to users.

LUNs can be shared between several servers; for example, between an active server and a failover server. But problems can arise if a number of servers access the same LUN at the same time. There needs to be a method of ensuring data integrity because blocks are subject to change by the activities of those servers. For this, you need something like a clustered volume manager, clustered file system, clustered application, or a network file system using NFS or CIFS.

The definition of a volume

LUN and volume are frequently used interchangeably. But it is worth noting that volume is also often used to describe groups of several LUNs created with volume manager software.

What is LUN masking?

LUN (Logical Unit Number) Masking is an authorization process that makes a LUN available to some hosts and unavailable to other hosts.

LUN Masking is implemented primarily at the HBA (Host Bus Adapter) level. LUN Masking implemented at this level is vulnerable to any attack that compromises the HBA.

Some storage controllers also support LUN Masking.

LUN Masking is important because Windows based servers attempt to write volume labels to all available LUN’s. This can render the LUN’s unusable by other operating systems and can result in data loss.

What is a HBA?

A HBA, or Host Bus Adapter, is the interface card which connects a host to a SAN (Storage Area Network).

A HBA could be more accurately referred to as a “Host I/O controller”.


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