Get to Know Clustered File Systems

“Some readers may wish to configure a cluster of servers that
simply have concurrent access to the same file system, while others
want to replicate storage and provide both concurrent access and
redundancy. There are two ways to go about providing multiple
servers access to the same disks: let them both see it, or via
replication.

“Shared-disk configurations are most common in the Fibre Channel
SAN and iSCSI worlds. It is quite simple to configure storage
systems such that multiple servers can see the same logical block
device, or LUN, but without a clustered file system, chaos will
ensue if both try to use it at the same time. This problem is dealt
with by using clustered file systems, which we will cover in a
moment.

“Generally speaking, shared-disk setups have a single point of
failure: the storage system. This is not always true, however, as
“shared-disk” is a confusing term with today’s technology. SANs,
NAS appliances, and commodity hardware running Linux can all
replicate the underlying disks in real-time to another storage
node, which provides a simulated shared-disk environment. Since the
underlying block devices are replicated, the nodes have access to
the same data and both run a clustered file system, but this
replication breaks the traditional shared-disk definition.”

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