Glossary

Storage area network (SAN)

A storage area network, or SAN, is a dedicated network that connects servers to shared storage arrays, so remote storage appears to each server as a local disk. The array carves out numbered volumes called LUNs and assigns them to hosts across a separate fabric.

What is a SAN made of?

The array creates numbered volumes called LUNs and assigns each one to a server, which formats it as its own disk. That makes a SAN the delivery network for block storage. It runs on its own network rather than the everyday LAN: Fibre Channel, iSCSI over Ethernet, or NVMe over Fabrics. The fabric switches in between, together with the array, give servers shared capacity with performance close to a local disk.

Zoning and masking

Without access control every server would see every volume. Zoning limits which ports on the fabric can talk to each other. Masking limits which hosts can see which LUNs on the array. Together they keep a database server's volume away from an unrelated host, and a mistake in either is how a volume gets damaged by the wrong machine.

What belongs on a SAN?

SANs are strong at small, frequent, latency-sensitive input and output: databases, business applications and virtual machine disks. That predictable behaviour is what the price pays for. Over the years, though, SANs also collect file shares, backups and archives that never needed block storage. Capacity there costs much more per terabyte than NAS or object storage, so the data that does not need speed is the cheapest space to reclaim.

A useful first step is an audit: what fraction of the allocated space really holds databases and VM disks. In many estates the answer is a minority, with the rest being data parked there because the SAN had room.

Why it differs from NAS

A SAN delivers raw volumes and leaves the file system to the server. A NAS delivers finished files and handles sharing. The distinction explains why a SAN cannot be mounted by many servers at once without extra software, and why moving data off it is a decision about workloads. Estates that combine many arrays follow SAN consolidation.

Redundancy is built in at every layer. Servers connect through two fabrics with separate switches, and multipathing software sends traffic down either path. A single cable or switch failure therefore causes no outage, which is part of why SANs earn trust for databases and why the fabric is treated as a system of its own.

Frequently asked questions

Is iSCSI a SAN?

Yes, when it runs on a network built for storage traffic. It carries block commands over Ethernet and costs less than Fibre Channel, though performance depends on how the network is designed.

What is a LUN?

A logical unit number identifies one volume the array presents to a host. A LUN can be larger or smaller than any one physical drive, since the array builds it from many.

Does a SAN replace a NAS?

No. They do different jobs. A SAN supplies disks to servers, while a NAS shares files with users and servers.