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Clustered storage essentials
Global file system All storage is pooled and made available to hosts through a single, scalable file system that hosts see as an ordinary drive letter.
Capacity scaling Disk capacity can be added incrementally and is automatically detected and merged into the storage pool; existing data is re-striped to take advantage of the new capacity.

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Performance scaling Performance scales with each added controller.
Load balancing The workload is shared among cluster members and redistributed as necessary.
Failover Redundant components with no single point of failure; transparent recovery from failure.
Single-pane management All controllers and storage installed in the cluster can be managed via one interface.

Basic foundation
Clustered storage systems owe their intelligence to the sophisticated software that controls operations. The underlying hardware is generally unremarkable--off-the-shelf parts such as Intel's Xeon processors and other standard components used in ordinary servers and server blades. In many cases, the cluster OS is built on top of a Linux kernel. This adds up to a low-cost architecture that provides the necessary economies for modularity.

Vendors of clustered storage hardware systems have managed to hide the inherent complexities and advanced features so effectively that most users first cite ease of use when describing what impresses them most about the clustered systems they've implemented. Users report that scaling with additional modules is as close to plug and play as storage gets. With no client software to install on hosts and streamlined, Web-based user interfaces that reduce many configuration and admin chores to point-and-click operations, implementation seemingly couldn't be easier.

"Most of the clustered storage systems ESG has analyzed are extremely easy to work with," says the firm's Asaro. Sports Illustrated's Jache concurs: "Honestly, the setup is 30 minutes," he says. "It's drop-dead easy."

Ron Godine, manager of IT operations at Glenwillow, OH-based Royal Appliances Manufacturing Co., the maker of Dirt Devil vacuums and other floor-care products, moved some data from an EMC Corp. Clariion to a LeftHand clustered array. In his search for a successor to the Clariion, he considered traditional and clustered storage systems. "Instead of a monolithic array, we wanted something that was more scalable," says Godine. For Royal, the LeftHand system has proven to be economical and easy to manage. "You can create and destroy LUNs in a fraction of the time it takes with other systems," says Godine.

Clustered storage system sampler
  • 3PAR InServ Storage Server
  • EqualLogic PS Series
  • Exanet ExaStore
  • Isilon IQ
  • LeftHand Networks SAN/iQ
  • Panasas ActiveScale Storage Cluster
  • Xiotech Magnitude 3D
  • HP StorageWorks Scalable File Share (HP SFS)
  • IBM SAN File System (SAN FS)
  • Ibrix Fusion
  • NetApp SpinServer cluster
  • PolyServe Matrix Server
  • Red Hat Global File System
  • SGI InfiniteStorage Shared Filesystem CXFS
  • Veritas Cluster Server
Best fit for clusters
The vendors in the vanguard of clustered storage largely found their way into data centers on the strength of how well their systems handle digital imaging applications, such as video editing and pre-production. For Sports Illustrated, Isilon's systems filled the bill. "This is digital data that only gets read a few times," notes Jache. "It's not like a database where you're doing a lot of incremental reads."

Some clustered storage vendors report that their customer rosters have grown beyond the entertainment and scientific industries to include financial, government, education and healthcare sectors. For example, Mark Rivard, network systems specialist at Johnson Memorial Hospital in Stafford Springs, CT, uses a 3TB EqualLogic PS array for file serving, e-mail and as a virtual disk for backup.

"We back up the entire domain to the cluster first and then to tape," says Rivard. And he plans to expand the number of applications using the system. "Every application that comes into the organization that has any volume of storage will be attached to [the EqualLogic cluster]," he says.

But despite its undeniable appeal, clustered storage is not necessarily a good fit for mission-critical database and online transaction processing applications. "Clustered architectures have to coordinate every request over the fabric to communicate with other nodes," says Sujal Patel, Isilon's CTO, chairman and founder, "and that coordination takes time because there's latency in GbE."

But Patel sees technologies such as Infiniband reducing that latency and increasing bandwidth between nodes. "The speed of the networking interface between the nodes is going to approach the speed of the computer bus; as that occurs, clustered architectures won't have any disadvantage vs. monolithic architectures," he says.

Clustered storage has a lot going for it. It effectively--and often elegantly--addresses some of the key drawbacks and bottlenecks associated with traditional storage systems. "It can greatly simplify the management of storage networks," says ESG's Asaro, and "when combined with intelligence such as moving data between tiers, data de-duplication, retention polices and meta data, it can create new [storage] applications."

Evaluator Group's Kerns is bullish on clustering, too: "It's the logical evolution," he says. "As more people catch on, it's going to become more and more popular."

This was first published in April 2005

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