A good example of a loosely coupled NAS is the OnStor NAS solution. In the cloud, storage area companies such as ParaScale offer a loosely coupled solution for building a private cloud. Each NAS head in loosely coupled solutions is essentially its own entity and isn't dependent on the other nodes in the cluster to stay up and running. Typically, each volume or file system is the property of a particular node in the cluster. It can be reassigned on the fly, but there is a one-to-one relationship at any given point in time. In the event of a node failure either through automated software or manually, a file system must be reassigned to another.
There are quite a few examples of tightly coupled clusters. Isilon offers a tightly coupled solution for primary storage, Cleversafe is an example of a tightly coupled solution in cloud storage, and Permabit is a good example in archive or retained storage. In a tightly clustered system, the volume or file system is distributed across every (or most) nodes in the cluster. These file systems are dependent upon other nodes in the cluster to stay up and running. Tightly coupled solutions offer some form of "RAID-like" redundancy so that a failed node doesn't result in data loss or even loss of access. Permabit, for example, uses a protection scheme called RAIN-EC that can survive multiple node failures while not requiring the redundant capacity that a mirror or triple-mirror protection scheme would require.
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George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.