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Clustered file system options

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The key difference between the two clustered file system options (NAS aggregation and SAN file systems) is the location of the cluster and the file system. NAS aggregation clusters NAS servers, while SAN file systems cluster host servers.

Clustered NAS
Small NAS filers have another glaring fault--single points of failure. The availability problem has long been met by clustered NAS systems like EMC's Celerra. This system breaks the link between an IP address and the "data movers" that serve client requests. Celerra acts like a traditional NAS system in most respects, but its ability to fail over one data mover to another in the event of a failure brought it credibility in the enterprise. Today, most NAS makers, including BlueArc Corp.,

NetApp, OnStor and others offer similar clustering abilities for high availability.

BlueArc's Titan NAS server is the standard-bearer for monolithic NAS. Rather than clustering small NAS arrays with a single namespace, BlueArc seeks to scale a single box to handle all of an enterprise's NAS needs, though it does include an internal cluster for high availability. "It will always be easier to manage one array than many small ones," contends Geoff Barrall, BlueArc's CTO.

By contrast, OnStor's SAN Filer is an integrated hardware and software platform that consolidates file services on open SAN storage.

Jon Toor, director of marketing for OnStor, notes that "workloads tend to be concentrated on a few cluster members." So OnStor's Filer uses virtual NAS servers that can be shifted to other physical filers on the fly to balance the load. All of the state information for each virtual filer is stored on the SAN disk, so the physical filers can be added and removed at will. Although this approach is likely to lead to a proliferation of virtual NAS servers and shares, it scales extremely well in terms of performance and availability. For a company that needs extremely high availability and wants to leverage SAN storage, but doesn't need a single client-side view of NAS shares, OnStor's SAN Filer is compelling.

Nearly all recent NetApp filers support clustered failover; however NetApp's SpinServer provides more than basic clustering. Although SpinServer is currently targeted at high-performance Linux clusters, SpinServer can also support general business needs. SpinServer shares files with traditional protocols like NFS and CIFS, and the entire cluster appears as a single system to both clients and managers.

What to choose?
Most NAS filers today offer high availability with failover pairs. But the ability to scale an environment requires much more than this. Considerations include scalability demands, dispersed storage and the difficulty of managing a multitude of files, shares and servers.

Almost all solutions can scale, both in terms of performance and capacity, with the addition of more hardware. But not all can seamlessly integrate this new equipment into the existing environment. This is the key challenge for products that do not offer a virtual namespace--new hardware appears as new network shares and users must change their behavior to use it.

To avoid this problem, consider a global namespace product such as NuView's StorageX or Z-force's File Switch; but remember, these are Windows only. The only way to implement a cross-platform global namespace is to replace your NAS infrastructure with, for example, NetApp's SpinServer or Panasas' ActiveScale. If Oracle 10 on a Linux cluster is in your future, then the NetApp and Panasas solutions should be on your short list.

Many of these solutions may simplify your users' view of storage, but they won't necessarily make your life easier. The router-style solutions from NuView and Z-force still require management of the underlying storage infrastructure. And the integrated solutions from BlueArc, NetApp, OnStor and Panasas are entirely new storage architectures with their own learning curve. Once you have mastered these technologies, though, management should be simple since they are all controlled through a single application.

Finally, if you don't want to bother with new clustering architectures, it's perfectly okay to stick to a more traditional NAS system with high levels of scalability. EMC's Celerra is offered in a number of configurations from the single-node NS700G NAS head and NS700 two-node cluster, to the 14-node CNS. NetApp has a range of clustered systems from the small FAS200 to the much larger FAS900. IBM Corp.'s NAS Gateways are another popular scalable NAS solution.

If you have a proliferation of NAS systems and mount points, or if you are concerned that your NAS solution isn't enterprise-class, this new breed of NAS products can help shield your users from the complexity of aggregating NAS. And these systems will also help ensure that you can offer NAS users the same levels of availability and scalability found in your SAN.

This was first published in June 2004

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