The simplest form of clustering involves two controller units paired so that one would provide failover for the other. In a two-way, active-passive paired configuration, one controller is essentially on standby. Because this scheme doesn't provide for scaling and the passive unit isn't sharing the primary's load, this is often referred to as "pseudo clustering." An active-active controller arrangement is a step up from pseudo clustering, where the two controllers provide failover for each other and share the work.
In a non-distributed, active-active cluster, cluster members share a file system and some other physical resources, but provisioning and LUN assignment for specific controllers are mostly manual chores. The distributed peer cluster is the most common architecture employed by vendors that have designed and built their clustered storage systems from the ground up. In a distributed cluster, physical resources are virtualized, so a storage administrator only needs to deal with how storage is associated with installed servers and the applications they host.
Single file system
There are products that provide global file system capabilities for aggregated storage systems like IBM's SAN File System (SAN FS). These applications typically run on an appliance or an intelligent switch with client software on supported hosts to deliver one of clustering's key requisites, the global file system.
SAN FS and similar products take a two-pronged approach: They virtualize the storage they sit in front of into a single file system and theyt interface with the hosts' OSes to present that file system as if it were native to the hosts. In this manner, these systems improve capacity management by providing policy-based data migration across all connected storage. That enables more effective storage tiering, a basic step toward an information lifecycle management implementation. SAN FS works with a variety of Windows, Linux and Unix hosts, but requires an IBM storage system for its meta data store. It supports numerous back-end storage systems and can be used in conjunction with IBM's SAN Volume Controller (SVC) to support a range of storage arrays.
Examples of other clustered file system products include Ibrix Inc.'s Fusion, PolyServe Inc.'s Matrix Cluster, Red Hat Inc.'s Global File System (formerly Sistina GFS), SGI's InfiniteStorage Shared Filesystem CXFS and Veritas Software Corp.'s Cluster Server. These are all host-based apps that cluster servers and provide a single image of the storage available attached on SANs.
Clustered file systems are attractive because they can work with installed storage. On the other hand, hardware clustering systems require the purchase of new storage (see sidebar on software vs. hardware clustering.)
But virtualization and a global file system don't necessarily add up to a fully clustered storage system. Randy Kerns, a senior partner at Evaluator Group Inc., Greenwood Village, Colo., describes SAN FS as "a meta data server approach to storage virtualization." It provides a key element, but it's only part of the clustering picture. "It's one way to provide a global namespace," noted Kerns, "but a global namespace and clustered storage are not necessarily connected."