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.
A typical DAS system is made of a data storage device (for example enclosures holding a number of hard disk drives) connected directly to a computer through a host bus adapter (HBA). Between those two points there is no network device (like hub, switch, or router), and this is the main characteristic of DAS.
The main protocols used for DAS connections are ATA, SATA, eSATA, SCSI, SAS, and Fibre Channel.
Storage features common to SAN, DAS and NAS
Most functions found in modern storage do not depend on whether the storage is attached directly to servers (DAS), or via a network (SAN and NAS).
A DAS device can be shared between multiple computers, as long as it provides multiple interfaces (ports) that allow concurrent and direct access. This way it can be used in computer clusters. In fact, most SAN-attachable storage devices or NAS devices can be easily used as DAS devices – all that is needed is to disconnect their ports from the data network and connect one or more ports directly to a computer (with a plain one-to-one cable).
More advanced DAS devices, like SAN and NAS, can offer fault-tolerant design in many areas: controller redundancy, cooling redundancy, and storage fault tolerance patterns known as RAID. Some DAS systems provide embedded disk array controllers to offload RAID processing from the server's host bus adaptor (HBA). Basic DAS devices do not have such features.
A DAS can, like SAN or NAS, enable storage capacity extension, while keeping high data bandwidth and access rate.
DAS has been referred to as "Islands of Information. The disadvantages of DAS include its 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.
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