Tightly coupled and loosely coupled clusters are two classifications of storage clusters that can overcome obstacles like capacity, storage I/O, and expandability. Tightly coupled clusters divide data at a granular level between nodes and see improved performance as more drives and nodes are added. However, their components must come from the same supplier and do not allow flexibility. Loosely coupled clusters have standalone dual controller systems with a global namespace but performance does not scale up as nodes are added like tightly coupled clusters since data resides on a single node. Both approaches can address storage problems but review which classification fits your specific storage demands.

1. Storage Clusters - Tightly Coupled vs.
Loosely Coupled
Clustered storage can overcome several key obstacles that storage professionals face today;
capacity, storage I/O and expandability. There are two classifications of storage clusters, loosely
coupled and tightly coupled and each have their place in the environment.
Tightly coupled clusters tend to be focused on primary storage, although it is available in
archive and it is even used in disk to disk backup systems. In a tightly coupled cluster the data is
divided up between nodes in the cluster at some level of granularity smaller than a file, typically
a block or sub block level. Unlike a typical dual controller architecture where performance peaks
at about half of drive capacity and then declines, a tightly coupled cluster should see improved
performance as more drives and nodes are added to the environment.
The downside is that the components of the cluster are more strict, most always come from the
supplier, don't allow for flexibility of mixing manufacturers or even tiers of systems from the
supplier. While some will use proprietary components with special capabilities, many will use
off the shelf Intel or Xyratex storage servers and then add their software plus some sort of
interconnect, typically ethernet, but some will use advanced interconnects like Infiniband.
Loosely coupled clusters are essentially stand alone dual controller systems with a global name
space that reduces the management of the environment. This has the advantage of greater
flexibility in the members of the cluster and that the cluster can start with just one member.
Unlike a tightly coupled cluster the data is not stored across nodes in the cluster, the entirety of
the file must be on a single node. It can however be mirrored to other nodes for redundancy and
some storage software will know to use the secondary copy as a backup automatically.
Loosely coupled clusters have the downside that maximum performance and capacity is limited
to the performance of the node that houses the data. The performance does not scale up as nodes
are added like a tightly coupled cluster does. As a result loosely coupled clusters tend to be
applied where performance is important but inexpensive capacity is more important. Cloud
storage being one of the more ideal candidates.
Both styles of clustered storage can address the problems of scale, I/O and expandability
and many suppliers may be able to blur the differences between the two by either driving
up dual controller performance or driving down storage costs. Review both technologies
and understand which one makes the most sense for what your storage demands are.