Article

Defining storage-area networks (SANs), network-attached storage (NAS) and unified storage

Antony Adshead, UK bureau chief, storage
Network-attached storage (NAS), storage-area networks (SANs) and unified storage (a combination of NAS and SAN approaches) have a common element: they provide data storage capacity that isn't directly attached to servers. They pool storage on arrays of drives that are accessed by and independent of servers.

NAS, SAN and unified storage differ depending on the network structure that servers and data storage use to connect to each other, as well as on the networking protocols each technology uses to deliver and receive data.

What is network-attached storage?

Before storage networking, computing environments used disks directly attached to servers to store data, while clients connected to servers over an Ethernet network using TCP/IP. Then Sun Microsystems developed the Network File System (NFS) that allowed data to be shared over an Ethernet network. NFS enabled local users to access files on a remote machine over a TCP/IP network and access it just as if it were on the local hard drive. Microsoft later created the Common Internet File System (CIFS) protocol to enable access in the same way for Windows computers.

Network-attached storage is a dedicated disk storage subsystem residing on a local-area network (LAN) that provides access to entire files using the CIFS (Windows) or NFS (Unix) protocols. NAS impacts LAN traffic and is generally seen as a solution for smaller businesses or remote offices who want to access whole files.

What is a storage-area network?

A storage-area network addresses essentially the same challenge as a network-attached storage system – to allow server access to a shared pool of data storage.

The critical difference between SAN and NAS is that SAN storage delivers data in blocks rather than whole files, which makes storage-area networks – especially Fibre Channel (FC) SANs – well-suited to delivering large amounts of transactional data at very high levels of I/O performance, such as for databases.

Storage-area networks and network-attached storage were once also distinguishable by the way they delivered data. SAN used to be defined by residing on a network discrete from the LAN – known as a fabric – upon which SCSI commands were encapsulated within the specially developed low latency Fibre Channel protocol.

But in recent years, the iSCSI protocol has emerged, which contains SCSI block storage commands within TCP/IP and can be used over the LAN/WAN. For that reason, SAN has come to mean block storage as opposed to file storage.

A key advantage of Fibre Channel SANs is that the storage network is entirely separate from the local-area network. This can bring performance benefits to data storage and backup that iSCSI may not be able to because of its use of Ethernet networks, which may be shared with other traffic. The downside to Fibre Channel is that the staff training overhead is far greater than for iSCSI, which uses technology protocols familiar to networking professionals.

What is unified storage?

Unified storage – also known as multiprotocol storage – is a single data storage system that supports both file and block access. The subsystem can support NAS (file-based access), as well as iSCSI and Fibre Channel (block-based access) storage protocols simultaneously.

Broadly speaking, there are two ways of achieving unified storage. You can buy arrays that are built as dedicated multiprotocol storage arrays with a NAS server and storage-area network controller combined under a single management interface. Or you can provide file-level access to a SAN disk array by adding a so-called network-attached storage head or gateway.

Dedicated unified storage systems are popular for small- to medium-sized enterprises (SMEs) and remote-office implementations where flexibility in how servers are attached to data storage is at a premium. The unified storage head/gateway approach is better suited to larger enterprises wanting more flexibility from their existing information technology investments. This approach may result in data storage managers having to manage two systems – the storage-area network array and the network-attached storage head.


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