LUNs

A LUN, or logical unit number, is a logical representation of a storage resource presented to a host in a storage area network or other block storage environment. In SCSI-based and modern SAN architectures, a LUN maps to a logical unit within a storage array and is exposed to the host as a block device. The host formats it with a file system or uses it as raw block storage, independent of the underlying disks or RAID layout.

LUNs are created on a storage array or controller and then exported to hosts over networks such as Fibre Channel or iSCSI. Each LUN is identified by a local LUN ID and is addressed by the host through a specific target or portal. The host sees it as a disk-like device and can attach it to a operating system or virtualization environment. LUNs can be shared with multiple hosts in some configurations, but access is controlled to maintain isolation and data integrity.

Access control mechanisms, such as LUN masking and zoning, restrict which hosts can see and use a LUN. LUN masking is commonly used in multi-tenant or multi-server environments to prevent unauthorized access, while zoning restricts access at the network level in Fibre Channel.

Provisioning and characteristics vary: LUNs can be thick-provisioned (pre-allocated) or thin-provisioned (on-demand). They have defined capacity, block size, and may support features such as snapshots, clones, or replication. LUNs may also be part of larger storage constructs, like volumes or aggregates, and can be managed with multipathing to provide redundant paths and improved reliability.