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.
