Xsan is a storage area network file system (ACFS, or Apple Cluster File System) and a management application (Xsan Admin) you can use to provide expandable storage to users or applications on client computers with shared high-speed access.
A storage area network (SAN) is a way of connecting computers to storage devices that gives users very fast access to files and gives administrators the ability to expand storage capacity as needed without interrupting users.
An Xsan SAN consists of:
Volumes of shared storage, stored on Apple Xserve RAID (Redundant Array of Independent Disks) systems, available to clients as mounted volumes that they can use like local disks
At least one computer acting as a metadata controller that coordinates access to the shared volumes
Client computers that access storage in accordance with established permissions and quotas
Underlying Fibre Channel and Ethernet networks.
The following illustration shows the physical components of a typical Xsan SAN.
Shared SAN volumes that you create in the Xsan Admin application will appear to the client as a single local volume. You can create these volumes by selecting a combination of RAID arrays to be included in a pool, which, in turn, is added to create the volume. Volumes can be up to 16 terabytes in size. This means that a client machine will be able to see up to eight 16 TB volumes on their desktop. Furthermore, clients can write one file that is 16 TB in size, or create up to 4 billion files and directories per volume!
You must assign at least one computer to become a controller when setting up your SAN. This machine has the job of maintaining the volume metadata of the SAN, file journaling, and concurrent access to files. This controller “remembers” which RAIDs are part of a pool, and how you have configured your pools to make volumes. Although the controller is in charge of these tasks, the actual data is stored on the SAN. In the event of failure of the main controller, Xsan will automatically switch to the backup controller. The backup controller is aware of how the SAN is configured, so if failure occurs and the backup kicks into action, clients are often unaware that anything has happened. In fact, a client machine can act as a backup controller when you have only one system available as a main controller.
Machines that have access to the SAN are called clients. You can have a total of 64 clients in Xsan with concurrent block level access to a shared volume. Clients are configured in the Xsan Admin application and are able to mount the shared storage if given access. The files the client reads and writes are sent via the Fibre Channel connection, but other client/controller communication is made possible by an Ethernet “out of band” network. An unlimited number of clients may indirectly access data from the SAN if any node is configured to host a network service such as file sharing (AFP, SMB/CIFS, and NFS).