DAS (Direct- Attached Storage)
Direct-attached storage (DAS) refers to a digital storage system directly attached to a server or workstation, without a storage network in between. A "retronym", it is the difference between non-networked storage versus SAN and NAS.(4)
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) with no network device in between (like hub, switch, or router), and this is the main characteristic of DAS. The main protocols used for DAS connections are ATA, SATA, eSATA1, SCSI, SAS, and Fibre Channel. Most functions in modern storage are flexible with directly attached storage to servers (DAS), or via a network (SAN and NAS).
A DAS device can be shared between multiple computers with multiple interfaces (ports) that allow concurrent and direct access allowing use in computer clusters. Most SAN-attachable storage devices or NAS devices can be easily used as DAS devices –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 (a network designed to attach storage hardware and software to servers-network between computer systems and storage systems, or as a complete system that includes all of the storage elements and computer systems within the same network) and NAS (the operating system and other software on the NAS product are dedicated solely to data storage), can offer fault-tolerant design: 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) but basic DAS devices do not include the embedded disk array controllers. A DAS can, like SAN or NAS, enable storage capacity extension and keep high data bandwidth and access rate.
Disadvantages
DAS has been referred to as "Islands of Information.(2) The disadvantages of DAS- inability to share data or unused resources with other servers. Both NAS (network-attached storage) and SAN (storage area network) architectures address this, but create other disadvantages- higher initial cost(3), manageability, security, and contention for resources.
References
(1) http://www.serialata.org/technology/esata.asp
(2) http://www.itmanagement.com/faq/storage-faq/
(3) "SAN vs. DAS: A Cost Analysis of Storage in the Enterprise". A Cost Analysis of Storage in the Enterprise. Capitalhead.com. 2008-11-03. Retrieved 2009-06-11.
(4) http://en.wikipedia.org/wiki/Direct-attached_storage
