2.2.3. Shared Storage: NAS and SAN
💡 First Principle: When multiple servers need access to the same storage, you need shared storage. The key distinction between NAS and SAN is the level of abstraction: NAS presents a file system (clients access files); SAN presents raw block storage (clients see a disk they format themselves). This determines what workloads each is suited for.
This distinction is one of the most commonly confused concepts on the Server+ exam—and in practice.
NAS (Network Attached Storage)
A NAS device is a dedicated file server accessible over the network. Clients mount shares and access files using file-sharing protocols:
- NFS (Network File System): Unix/Linux clients. Lightweight, stateless, widely supported.
- CIFS (Common Internet File System): Windows clients; also known as SMB (Server Message Block). Supports Windows permissions and Active Directory integration.
NAS use cases: Shared home directories, departmental file shares, media storage, backup targets, collaborative document storage. Any scenario where multiple clients need access to the same files and file-level permissions matter.
SAN (Storage Area Network)
A SAN presents raw block storage over a high-speed network. The server sees a SAN LUN (Logical Unit Number) as if it were a directly attached disk—it must format the disk itself with a file system.
SAN protocols:
- iSCSI: Carries SCSI commands over TCP/IP networks. Lower cost (uses existing Ethernet infrastructure). Higher latency than Fibre Channel.
- Fibre Channel (FC): Dedicated high-speed storage network (2, 4, 8, 16, 32 Gbps). Lower latency, higher reliability, higher cost. Requires dedicated FC switches and HBAs (Host Bus Adapters).
- FCoE (Fibre Channel over Ethernet): Carries FC traffic over 10 GbE or faster Ethernet, reducing infrastructure cost while maintaining FC protocol compatibility.
NAS vs. SAN Comparison
| Dimension | NAS | SAN |
|---|---|---|
| Abstraction level | File level (shares/directories) | Block level (raw disk) |
| Protocols | NFS, CIFS/SMB | iSCSI, Fibre Channel, FCoE |
| Infrastructure | Standard Ethernet | Dedicated FC fabric or Ethernet |
| Cost | Lower | Higher |
| Performance | Adequate for file sharing | High IOPS, low latency |
| Typical use case | File shares, home directories | Databases, VM datastores, clustered apps |
| Multiple server access | Built-in (file sharing) | Requires cluster-aware file system (VMFS, GFS) |
SAN Identifiers
SAN devices are identified by unique World Wide Names (WWN):
- WWNN (World Wide Node Name): Identifies the storage device itself
- WWPN (World Wide Port Name): Identifies each individual port on the device
These are analogous to MAC addresses in Ethernet networking.
⚠️ Exam Trap: Multiple servers cannot mount the same SAN LUN simultaneously using a standard file system—concurrent access would corrupt the file system. Shared SAN access requires a cluster-aware file system (like VMware VMFS for virtualization or OCFS2 for Linux clustering). This is a frequently tested distinction.
Reflection Question: A virtualization administrator needs 20 VMware ESXi hosts to share the same storage pool for VM live migration. Should they use NAS or SAN, and which protocol would minimize infrastructure cost?
