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NAMING The term did not exist before the advent of networked Enterprise Storage , though it is often confused with the Mainframe term, '' DASD '', which refers to any Random-access Disk Storage system. Another common misconception is that DAS is synonymous with '' JBOD '', a term that refers to storage which is not protected with RAID but actually has no connectivity implications. FEATURES The main protocols used in DAS are SCSI and Fibre Channel . Traditionnaly, DAS systems enable storage capacity extension for servers, while keeping high data Bandwidth and access rate. Most common DAS Systems are made of one ore more enclosures holding storages devices such as Hard Disks , and one ore more controllers. Interfacing with the server or the workstation is made through the HBA (Host Bus Adapter). DAS systems typically offer fault tolerance at every level with 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 acccess 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. DISADVANTAGES DAS has been referred to as "Islands of Information". Disadvantages of DAS include inability to share data or unused resources with other servers. Network-attached Storage (NAS) attempts to address this issue, but introduces some new issues as well, such as cost of storage area networks, manageability, security, and contention for resources. |
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