NAS, SAN and Software-defined storage are the primary methods available today to supply shared, networked storage to servers. Shared storage can be partitioned and used by multiple applications and, as such, can amortize the cost of storage across multiple users.
NAS, or Network Attached Storage, is normally attached over a Local Area Network (LAN) using Ethernet but can use other physical transports such as InfiniBand. NAS systems present file storage to a host server. NAS data is accessed as a sequence of data enclosed inside a file that itself is located inside a directory within a file system. File systems are mounted for UNIX®/Linux® and are shared under Windows®. All NAS systems are accessed using LAN Network Interface Cards (NICs), InfiniBand HCA (host channel adapters) or similar, on motherboard functionality.
Somewhat complicating our discussion about NAS and SAN storage is the fact that file systems can be readily created out of block storage. In contrast, NAS systems don’t normally supply block services. Nonetheless, some NAS systems also support block storage protocols and, as such, can provide unified storage or file and block storage from the same storage system.
SAN is an acronym for Storage Area Network and traditionally meant FC (Fibre Channel) attached, networked storage system but now can also mean FCoE (Fibre Channel over Ethernet) attached storage or IP storage area network (IP/SAN) attached storage using iSCSI (Internet Small Computer System Interface). SAN storage systems present block storage to a host server. Block storage is accessed as a collection of numbered blocks within a Logical Unit Number (LUN or volume) for UNIX/Linux or a drive letter under Microsoft® Windows. SAN storage is connected to a server via a Host Bus Adaptor (HBA) for FC SAN, a Converged Network Adaptor (CNA) for FCoE SAN or a NIC for IP/SAN storage.
Software-defined storage is explained more fully below but can be either file or block but mostly supports block storage. Software-defined storage is scale-out storage and operates on customer commodity severs and uses directly attached server storage, with or without virtualization. These systems generally require 3 or more nodes and use the LAN to share storage accessed via NICs.