Storage area network

In computing, a storage area network (SAN) is a network designed to attach computer storage devices such as disk array controllers and tape libraries to servers. As of 2006, SANs are common in enterprise storage.

There are two variations of SANs:

A network whose primary purpose is the transfer of data between computer systems and storage elements. A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust. The term SAN is usually (but not necessarily) identified with block I/O services rather than file access services.
A storage system consisting of storage elements, storage devices, computer systems, and/or appliances, plus all control software, communicating over a network.

Defining SAN

Storage networks are distinguished from other forms of network storage by the low-level access method that they use. Data traffic on these networks is very similar to those used for internal disk drives, like ATA and SCSI.

In a storage network, a server issues a request for specific blocks, or data segments, from specific disk drives. This method is known as block storage. The device acts in a similar fashion to an internal drive, accessing the specified block, and sending the response across the network.

In more traditional file storage access methods, like SMB/CIFS or NFS, a server issues a request for an abstract file as a component of a larger file system, managed by an intermediary computer. The intermediary then determines the physical location of the abstract resource, accesses it on one of its internal drives, and sends the complete file across the network.

Most storage networks use the SCSI protocol for communication between servers and devices, though they do not use its low-level physical interface.

Benefits

Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to move storage from one server to another. Note, though, that with the exception of SAN file systems and clustered computing, SAN storage is still a one-to-one relationship. That is, each device (or Logical Unit Number (LUN)) on the SAN is "owned" by a single computer (or initiator). In contrast, Network Attached Storage (NAS) allows many computers to access the same set of files over a network. It is now possible to combine the SAN and NAS using a NAS head, such as the gateway manufactured by ONStor

SANs tend to increase storage capacity utilization, since multiple servers can share the same growth reserve.

Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. This process can take as little as half an hour and is a relatively new idea being pioneered in newer data centers.

SANs also tend to enable more effective disaster recovery processes. A SAN attached storage array can replicate data belonging to many servers to a secondary storage array. This secondary array can be local or remote.

SAN types

SANs are normally built on an infrastructure specially designed to handle storage communications. Thus, they tend to provide faster and more reliable access than higher level protocols such as NAS.

The most common SAN technology is Fibre Channel networking with the SCSI command set. A typical Fibre Channel SAN is made up of a number of Fibre Channel switches which are connected together to form a fabric or network.

An alternative, and more recent (2003), SAN protocol is iSCSI which uses the same SCSI command set over TCP/IP (and, typically, Ethernet). In this case, the switches would be Ethernet switches.

One alternative to iSCSI is the ATA-over-Ethernet or AoE protocol which embeds the ATA protocol inside of raw Ethernet frames. While a raw Ethernet protocol like AoE cannot be routed without something else performing the encapsulation, it does provide a simple discovery model with low overhead.

Connected to the SAN will be one or more servers(hosts) and one or more disk arrays, tape libraries, or other storage devices. In the case of a Fibre Channel SAN, the servers would use special Fibre Channel host bus adapters (HBAs) and optical fiber. iSCSI SANs would normally use Ethernet network interface cards, and often specialized TOE cards.

Storage area networks are of two kinds - centralized storage area networks and distributed storage area networks

Compatibility

One of the early problems with Fibre Channel SANs was that the switches and other hardware from different manufacturers were not entirely compatible. Although the basic storage protocols (such as FCP) were always quite standard, some of the higher-level functions did not interoperate well. Similarly, many host operating systems would react badly to other OSes sharing the same fabric. Many systems were pushed to the market before standards were finalized and vendors innovated around the standards.

The combined efforts of the members of the Storage Networking Industry Association (SNIA) improved the situation during 2002 and 2003. Today most vendor devices, from HBAs to switches and arrays, interoperate nicely, though there are still many high-level functions that do not work between different manufacturers' hardware.

The HBAs must match with appropriate ID or connectivity is denied.

SANs at work

SANs are primarily used in large scale, high performance enterprise storage operations. It would be unusual to find a Fibre Channel disk drive connected directly to a SAN. Instead, SANs are normally networks of large disk arrays. SAN equipment is relatively expensive, therefore, Fibre Channel host bus adapters are rare in desktop computers. The nascent (as of 2004) iSCSI SAN technology is expected to produce cheaper SANs, but it is unlikely that this technology will be used outside the enterprise data center environment with the exception of remote replication partner sites. The remote replication partner sites enables the data center environment to exist in multiple locations for disaster recovery and business continuity purposes.

SANs in a SOHO

With the increasing rise of digital media in all phases of life and its effect on storage needs, it's natural that SANs will enter into the SOHO market. Currently this is dominated by NAS type of systems.

The SOHO scenery is becoming the main market for SAN infrastructures. The reasons are explained here:

Production systems like real-time film scanners and real-time postproduction applications require very high bandwidths that can not be provided by traditional file servers. For example, the standard "2K" film format requires more than 300MB/sg for each real-time stream, and several of these streams can be required simultaneously. As a result, several "Gigabytes" per second can be required, which means a huge problem for standard NAS topologies. In addition, these systems need to work with the same files collaboratively, so they can not be distributed through different file servers nor DAS connections.

The SOHO market has two main differences against most other IT markets:

It requires very high bandwidth for sequential read/write operations, but random access speed is not relevant.

Access permissions are not specially relevant.

This kind of applications can not take advantage of the typical NAS advantages against SAN topologies ("smart" RAM caches for random access databases, complex security schemes, hardware costs optimized for one-to-many topologies...). Instead of having many computers connected to the network (with each one requiring a low bandwidth and only the server being stressed under heavy traffic) the SOHO "real-time" area only needs to integrate a few systems, but all of them require a very big bandwidth to access to the same files. These issues are very well suited in SAN infrastructures, where the cost-per-connection is very high (in comparison with traditional networks), but where the aggregated bandwidth for sequential I/O operations is extremely high.

But even in the SOHO companies, this situation only happens in the "real-time area", which only contains a few systems (typically less than 20 systems). All other systems are connected through "Bridge" servers, which mount the SAN volumes in one side and export them to the standard network in the other side, acting as if they were a second layer of NAS systems.

As a result, most SOHO companies have a SAN/NAS mixed topology.

External links

Introduction to Storage Area Networks - Exhaustive Introduction into SAN, IBM Redbook

Local area networks | Computer storage | Computer buses

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