page_title: Storage area network
section_title: Storage layer
hierarchical_section_title: Storage area network / Components / Storage layer
caption_attribution_description: English: This illustration shows the Fibre Channel layers from the physical layer to the upper level protocols.
context_page_description: A storage area network or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access storage devices, such as disk arrays and tape libraries from servers so that the devices appear to the operating system as direct-attached storage. A SAN typically is a dedicated network of storage devices not accessible through the local area network.
  Although a SAN provides only block-level access, file systems built on top of SANs do provide file-level access and are known as shared-disk file systems.
context_section_description: On top of the Fibre Channel-Switched Protocol is often the serialized Small Computer Systems Interface (SCSI) protocol, implemented in servers and SAN storage devices. It allows software applications to communicate, or encode data, for storage devices. The internet Small Computer Systems Interface (iSCSI) over Ethernet and the Infiniband protocols may also be found implemented in SANs, but are often bridged into the Fibre Channel SAN. However, Infiniband and iSCSI storage devices, in particular, disk arrays, are available.
  The various storage devices in a SAN are said to form the storage layer. It can include a variety of hard disk and magnetic tape devices that store data. In SANs disk arrays are joined through a RAID, which makes a lot of hard disks look and perform like one big storage device. Every storage device, or even partition on that storage device, has a logical unit number (LUN) assigned to it. This is a unique number within the SAN and every node in the SAN, be it a server or another storage device, can access the storage through the LUN. The LUNs allow for the storage capacity of a SAN to be segmented and for the implementation of access controls. A particular server, or a group of servers, may, for example, be only given access to a particular part of the SAN storage layer, in the form of LUNs. When a storage device receives a request to read or write data, it will check its access list to establish whether the node, identified by its LUN, is allowed to access the storage area, also identified by a LUN. LUN masking is a technique whereby the host bus adapter and the SAN software of a server restrict the LUNs for which commands are accepted. In doing so LUNs that should in any case not be accessed by the server are masked. Another method to restrict server access to particular SAN storage devices is fabric-based access control, or zoning, which has to be implemented on the SAN networking devices and the servers. Thereby server access is restricted to storage devices that are in a particular SAN zone.
caption: Fibre Channel is a layered technology that starts at the physical layer and progresses through the protocols to the upper-level protocols like SCSI and SBCCS.
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