Middleware News

storage area network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries, and optical jukeboxes) to servers in such a way that the devices appear as locally attached to the operating system. Although the cost and complexity of SANs are dropping, they are still uncommon outside larger enterprises. Network attached storage (NAS), in contrast to SAN, uses file-based protocols such as NFS or SMB/CIFS where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block.

Network types Most storage networks use the SCSI protocol for communication between servers and disk drive devices. They do not use SCSI low-level physical interface (e.g. cables), however, as its bus topology is unsuitable for networking. A mapping layer to other low-level protocols is used to form a network: * ATA over Ethernet (AoE), mapping of ATA over Ethernet, * Fibre Channel Protocol (FCP), the most prominent one, is mapping of SCSI over Fibre Channel (FC), * Fibre Channel over Ethernet (FCoE), * mapping of FICON over FC, used by mainframe computers, * HyperSCSI, mapping of SCSI over Ethernet, * iFCP[1] or SANoIP[2] mapping of FCP over IP. * iSCSI Extensions for RDMA (iSER), mapping of iSCSI over InfiniBand (IB), * iSCSI, mapping of SCSI over TCP/IP

Storage sharing Historically, data centers first created "islands" of SCSI disk arrays. Each island was dedicated to an application, and visible as a number of "virtual hard drives" (i.e. LUNs). Essentially, a SAN connects storage islands together using a high-speed network, thus allowing all applications to access all disks. Operating systems still view a SAN as a collection of LUNs, and usually maintain their own file systems on them. These local file systems, which cannot be shared among multiple operating systems/hosts, are the most reliable and most widely used. If two independent local file systems resided on a shared LUN, they would be unaware of this fact, would have no means of cache synchronization and eventually would corrupt each other. Thus, sharing data between computers through a SAN requires advanced solutions, such as SAN file systems or clustered computing. Despite such issues, SANs help to increase storage capacity utilization, since multiple