Image Processing Reference
Storage-area network (SAN) systems have been evolving for several years, and they rep-
resent the current state of the art for high-end workgroup storage. A SAN is fast, low
latency, and shareable across a few dozen people (any more than that and you start to get
traffic problems). Where the performance improvement has been added is in the area of
metadata. Figure 29-3 shows how these are all connected together.
If a SAN is properly designed and implemented, it will look to the workstation user
as if he or she is accessing an internally mounted, direct-attached drive even though the
drive is remotely coupled by optical fiber.
A SAN requires a fiber-channel switch in order to connect it to the workstations and
get the required throughput. This is fundamental to getting it working; trying to save
money here is a false economy. The switch is easily the most expensive part of the whole
Normally, you enquire about the state of a file system from the network-attached
server. This is the same place that the files are stored. The SAN architecture delegates a lot
of this work to a secondary node called a metadata controller. This SAN file system cata-
log is stored here and is accessible quickly because the network is not saturated with file
I/O. The client talks to the metadata controller, finds out where on the SAN disk the files
are stored, and requests the physical file space via the fiber-channel link.
Other positive aspects of SAN systems include the following:
Backup can be built into the SAN back end.
Metadata controllers can be backed up with a secondary controller—the cost is
incremental and you are already spending a lot.
Fibre channel switch
SAN disk store
SAN metadata controller
Figure 29-3 SAN systems' organization.