Direct Attached Storage or DAS is a storage device that connects directly to a single server. In order for clients on the network to use the storage device they must have access to the server it is connected to. DAS units are a great way to add storage to an existing server. Each unit has a built-in RAID controller which supports various RAID levels and is easily configured via software or an LCD interface. When attached to the server, the storage unit is seen as a single drive. Added storage capacity is virtually plug and play!
The server not only handles the storage and retrieval of data files, but also handles applications such as e-mail or databases. As a result the performance of direct storage is not as high as that of networked storage. However, on a small scale, direct storage is much simpler to administer, as only one server connected to the storage device has to be dealt with.
RAID provides data redundancy, which is critical for servers. If a drive fails, you can replace it without having to turn off the system, and the DAS unit will rebuild the data automatically. Typical Applications for Direct Attached Storage Subsystems: AV Storage, Clustering, Enterprise High Performance Systems, and Disk-to-Disk Backup