Storage area network
A storage area network (SAN) is a storage device (such as disk arrays, tape libraries, and optical jukeboxes) accessible to servers so the devices appear as locally attached to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the regular network by regular devices. The cost and complexity of SANs dropped in the late 2000s, allowing wider adoption across both enterprise and small to medium sized business environments.
A SAN alone does not provide the "file" abstraction, only block-level operations. However, file systems built on top of SANs do provide this abstraction, and are known as SAN filesystems or shared disk file systems.
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[edit] Storage sharing
Historically, data centers first created "islands" of SCSI disk arrays as direct-attached storage (DAS), each dedicated to an application, and visible as a number of "virtual hard drives" (i.e. LUNs). Essentially, a SAN consolidates such storage islands together using a high-speed network.
Operating systems maintain their own file systems on them on dedicated, non-shared LUNs, as though they were local to themselves. If multiple systems were simply to attempt to share a LUN, these would interfere with each other and quickly corrupt the data. Any planned sharing of data on different computers within a LUN requires advanced solutions, such as SAN file systems or clustered computing.
Despite such issues, SANs help to increase storage capacity utilization, since multiple servers consolidate their private storage space onto the disk arrays.
Common uses of a SAN include provision of transactionally accessed data that require high-speed block-level access to the hard drives such as email servers, databases, and high usage file servers.
[edit] SAN and NAS
Network-attached storage (NAS), in contrast to SAN, uses file-based protocols such as NFS or SMB/CIFS where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block. Recently,[when?] the introduction of NAS heads[clarification needed] has allowed easy conversion of SAN storage to NAS.
[edit] SAN-NAS hybrid
Despite the differences between SAN and NAS, it is possible to create solutions that include both technologies, as shown in the diagram.
[edit] Benefits
Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to shift storage from one server to another.
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. There are a number of emerging products designed to facilitate and speed this up still further.Labels: NETWORK
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