A SAN is a system in which disk subsystems and servers both plug into a
dedicated network, a special low-latency, high-speed network optimized for
storage protocols. Any server can attach to any storage system—at least as
access controls permit. What servers can attach to is a storage volume that
has been defined and is referred to by its logical unit number (LUN). A LUN
might be a disk, a slice of a RAID 5 group, an entire storage chassis, or anything
the storage systems make available. Servers access LUNs on the block
level, not the file system level. Normally, only one server can attach to a
particular LUN at a time; otherwise, servers will get confused as one system
updates blocks without the others realizing it. Some SAN systems provide
cluster file systems, which elect one server to arbitrate access so that multiple
servers can access the same volume simultaneously. Tape backup units
can also be attached to the network and shared, with the benefit that many
servers share a single expensive tape drive. Another benefit of SANs is that
they reduce isolated storage. With DAS, some servers may be lacking free
disk space while others have plenty. The free space is not available to servers
that need it. With SAN technology, each server can be allocated volumes as
big as they need, and no disk space is isolated from being used.