Accusys ExaSAN SW-08 - Manual (Page 53)

6. Xsan
User Guide Page 6-2
6.2.1 Shared Xsan Volume
A user or application on a client computer accesses shared network Xsan volumes, just like they would
with a local volume. Xsan volumes are logical disks made up of storage pools of LUNs. The elements
which you combine to create an Xsan volume are described in Section 6.3 “How Xsan Storage Is
Organized” and Section 6.4 “Xsan Storage Breakdown.”
6.2.2 Metadata Controller (MDC)
When you set up an Xsan, you assign at least one client computer to act as the Metadata Controller. Note
that all MDCs require Mac OS X Server operating system. 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, an enterprise SAN usually includes more than one controller. If the
primary controller fails, the standby controller takes over. Given that the ExaSAN Compact is designed to
support up to either 4 or 8 clients, we recommend that two metadata controllers be configured. You may
choose to use a MDC as a video client in the workgroup at the same time.
6.2.3 Clients
The computers that users or applications use to access SAN volumes are called Clients. Clients exchange
metadata with the metadata controller over the private Ethernet network. However, they access SAN over
the PCIe connections, sending and retrieving file data to and from the RAID systems that provide storage
for the volumes. Note that a custom QSFP HBA (Quad Small Form Factor Pluggable Host Bus Adapter)
is required on each client, which enables the PCIe connectivity.
6.2.4 Network Connections
Xsan uses the following independent networks to connect storage devices, metadata controllers, and client
computers:
1. SAN over PCIe. Xsan moves data between clients and Xsan volumes over high-speed PCIe
connections. The controller also uses a PCIe connection to move metadata to and from the volume.
Xsan can take advantage of multiple PCIe connections between clients and storages.
2. Ethernet. Xsan controller and clients exchange file system metadata over a separate, private Ethernet
network. Xsan private (metadata) Ethernet network should be separate from public (Internet) Ethernet
network to prevent traffic from interfering with metadata communications.
6.2.5 QSFP (Quad Small Form-factor Pluggable) HBA (Host Bus Adapter)
When installed on the clients, this adapter provides each client the necessary PCIe interface to perform
reads and writes with the Xsan volumes.
6.3 How Xsan Storage Is Organized
Although an Xsan volume mounted on a client computer looks like a single disk, it consists of multiple
physical disks combined on several levels using RAID techniques. The following illustration shows an
example of how HDD modules in three ExaSAN PCIe RAID systems (configured by ExaSAN Quick
Setup, refer to Section 4.2.2.1, “Quick Setup”) are combined into a volume, which provides the disk space
that users see as one single large Xsan volume.