A clustered file system consists of a file system and volume manager installed across multiple application server nodes, says Greg Schulz, founder and senior analyst at StorageIO Group, Stillwater, MN. It creates a single logical data view, allowing any node to access data regardless of its location. Providing this shared access requires coordination among server nodes to prevent access conflicts and ensure data integrity. This approach is the foundation of NAS clustering, in which a file system is installed across multiple industry-standard servers.

A clustered file server, by contrast, consists of multiple NAS servers working as a single storage and file space instance, says Schulz. This creates a single, cohesive file system in which any node can access the file system as if it were running on one large physical NAS server, with data access coordination done at the individual file server with no need for communication among the nodes. Storage clustering isn't the mere presence of dual CPUs, controllers or even NAS heads in an active-passive configuration with the second component on standby in the event the primary component fails, notes Schulz.

Clustered storage is often a good fit for high-performance computing (HPC) apps in which hundreds, thousands or tens of thousands of clients read and write data to extremely large data sets. But much of the current focus on HPC is on its use for everyday business apps such as databases and messaging systems.

Various clustered storage systems also differ by the protocols or file systems they support, as well as the levels at which they store and serve data. According to StorageIO Group's Schulz, clustered storage systems that move data at the block level using the iSCSI protocol include 3PAR Inc., EqualLogic Inc. (now part of Dell Inc.) and LeftHand Networks Inc. Clustered systems from BlueArc Corp., Ibrix Inc., Isilon Systems Inc., Network Appliance Inc., ONStor Inc., Panasas Inc. and PolyServe (now owned by HP) support the NFS file system. Sun has made Lustre a key part of its clustering strategy, especially for HPC apps, and has pledged to support it on Linux, its own Solaris OS and on multivendor hardware.

Many clustered file systems only update meta data across all the storage nodes to reduce network bandwidth requirements, and to make it easier for large numbers of clients to access data concurrently.