Direct-attached storage

From Wikipedia, the free encyclopedia

This article does not cite any references or sources. (June 2008)
Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

Direct-attached storage (DAS) refers to a digital storage system directly attached to a server or workstation, without a storage network in between. It is a retronym, mainly used to differentiate non-networked storage from SAN and NAS.

[edit] Features

The main protocols used in DAS are SCSI, SAS, and Fibre Channel. Traditionally, a DAS system enables storage capacity extension for a server, while keeping high data bandwidth and access rate. A typical DAS system is made of one or more enclosures holding storage devices such as hard disk drives, and one or more controllers. The interface with the server or the workstation is made through a host bus adapter (HBA).

A DAS system typically offers fault-tolerant design in many areas: controller redundancy, cooling redundancy, and storage fault tolerance patterns known as RAID. Entry-level DAS systems most often are made of an enclosure without active components such as controllers, thus access logic and fault tolerance patterns are provided by the server HBA.

Middle and top range DAS systems provide embedded controllers. RAID management is off-loaded, and simple non-RAID HBAs can be used, lowering costs. DAS controllers also enable shared storage access, which permits multiple servers (usually no more than 4) to access the same logical storage unit, a feature that is mainly used for clustering. At this point, top range DAS systems share similarities with entry-level SAN systems.

[edit] Disadvantages

DAS has been referred to as "Islands of Information". Disadvantages of DAS include inability to share data or unused resources with other servers. Both NAS (network-attached storage) and SAN (storage area network) architectures attempt to address this, but introduce some new issues as well, such as higher initial cost, manageability, security, and contention for resources.