About Xsan SANs in macOS Server
A Storage Area Network (SAN) is a way of connecting computers and storage devices so computers have fast, shared access to files while making it easy for administrators to expand storage capacity.
An Xsan SAN consists of:
Shared data volumes
RAID systems that provide storage space that’s protected from disk failure
At least one computer acting as a metadata controller that combines RAID arrays and presents their storage to clients as volumes that behave like local disks
Client computers that access storage in accordance with established permissions and quotas
Underlying Fibre Channel, Distributed LAN Client (DLC), and Ethernet networks
Shared SAN volumes
A user or app on a client computer accesses shared SAN storage as if it were a local volume. Xsan volumes are logical disks made up of pools of RAID arrays.
When you set up an Xsan SAN, you assign at least one computer to act as the metadata controller. The controller manages volume metadata, maintains a file system journal, and controls concurrent access to files. Metadata includes such information as where files are stored and what portions of available storage are allocated to new files.
To guarantee volume availability, a SAN should include more than one metadata controller. If the active controller fails, a standby controller takes over.
The computers that users or apps use to access SAN volumes are called clients. Clients exchange metadata with controllers over an Ethernet network, but use Fibre Channel or Distributed LAN Client (DLC) to send and retrieve file data to and from the RAID systems that provide storage for the volumes.
Fibre Channel network connection
Xsan moves data between clients and SAN volumes over high-speed Fibre Channel connections. Controllers also use a Fibre Channel connection to move metadata to and from the volume.
Xsan can take advantage of multiple Fibre Channel connections between clients and storage. Xsan can alternate between connections for each read and write, or it can assign each RAID array in a volume to a connection when the volume is mounted.
Ethernet network connection
All versions of Xsan exchange file system metadata over an Ethernet network. (Controllers use Fibre Channel to read and write metadata on a volume.)
To prevent Internet or intranet traffic from interfering with metadata communications, you can optionally set up separate public (Internet) and private (metadata) Ethernet networks. To learn more, see the Apple Support article Use a private network for Xsan metadata in macOS Server.
Distributed LAN Client (DLC) network connection
Xsan 4.1 or later on OS X 10.11 or later supports StorNext’s Distributed LAN Client (DLC) network connection, which allows client connections to your SAN over Ethernet instead of a fibre connection. Connections for such tasks as ingesting and editing can take place over fibre, while DLC connections can be used for other tasks.
You can have a public network (Internet), a private network (metadata), and a DLC network all as independent networks, with optimum performance. You could instead have all three over a single wired Gigabit Ethernet connection, but throughput won’t be optimum.
Note: macOS only supports being a client using Distributed LAN Client (DLC). Being a metadata controller isn’t supported.