Simple definition
A LUN (Logical Unit Number) is a logical block-storage unit presented to a server, hypervisor, or cluster.
Technical definition
A SAN array exposes capacity as logical volumes that can be accessed through block protocols such as iSCSI or Fibre Channel. The LUN is mapped to authorized hosts, which see it as a block device. The operating system or hypervisor can then create a filesystem or datastore on it.
How it works / role
Its role is to separate and present storage capacity to selected hosts while the storage array manages redundancy, performance, and data protection underneath. Concurrent access by multiple hosts must use a cluster-aware design.
What is it used for?
Provide shared block storage to virtualization clusters, databases, servers, or applications that require block-level access.
Practical example
A storage array presents a 5 TB LUN to three ESXi hosts over iSCSI. The hosts use multipathing and create a shared VMFS datastore on that LUN.
Common issues
- LUN mapped to the wrong initiator or host group
- Incomplete multipathing or degraded storage paths
- LUN expanded on the array but not rescanned or extended at the host level
- LUN removed without a clean unmount or datastore procedure
Related terms
Key takeaway: Before changing a shared LUN, verify host mapping, multipathing, and the filesystem or datastore that consumes it.