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Aggregating NAS
There are two key solutions for NAS aggregation: First, some aggregation products offer a global namespace--consolidating all files into a single huge tree. And second, some offer clustering.

The scalability solution is attained by breaking the hard link between the file's network name and its actual location. For instance, NetApp's SpinServer allows files and whole directory trees to be moved between nodes in the background, even while clients are actively accessing them. This allows a SpinServer cluster to be scaled seamlessly to add new storage and redistribute current storage with no availability outages. By contrast, OnStor Inc.'s SAN Filer creates a number of virtual NAS servers containing all of the shares on the network.

If files and directories are no longer linked to their actual "homes," a single large virtual tree of directories can be created and maintained. Global namespaces combine independent file servers into a single virtual one. Even if users don't need a single namespace (see "

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Roll Your Own NAS cluster"), a virtual namespace can still allow scalability and availability. Most importantly, it allows the NAS directory tree to look how you want it to look, whether that means a single corporate tree or a number of departmental ones.

Global namespace
Global namespace is a type of meta-directory of NAS namespaces that allows storage administrators to automatically move and manage data across heterogeneous NAS environments as if they were a single filer.

An interesting global namespace product is NuView Inc.'s StorageX. It manipulates the Windows Active Directory to make any CIFS-based NAS servers--including Windows servers, NetApp Filers and anything else--appear to be a single tree. NAS clients just see a single huge NAS server, and can mount any part of the combined directory tree.

StorageX is software that leverages the Windows DFS technology, which is analogous to DNS for IP addressing. When a Windows host requests access to a file, StorageX transparently redirects the request to the NAS system that hosts the directory containing the file. For this reason, StorageX currently works only for Windows clients, but NuView promises an NFS implementation around September of this year. However, the NFS version might require a remount when the network address of the NAS system changes. And even some Windows applications, including Microsoft Exchange (the ever-present monster of the data center), can't handle the retries needed when the storage topology changes, so it requires a remount as well.

Another product in the global namespace party is Z-force's ZX-1000 File Switch. Rather than leveraging network services or building their own NAS filer, Z-force offers a file switch that sits between NAS clients and servers. The box combines the NAS resources behind them into a single namespace, enabling high availability and flexibility. Z-force File Switches can be installed in groups, allowing their performance to scale with user demands. These systems, too, are Windows only.

Not every global namespace exists outside the NAS filer, though. NetApp's SpinServer implements the namespace within a cluster of NAS servers so it can support any protocol, and client retries are not required. But a complete "forklift upgrade" of the NAS environment isn't desirable, so NetApp also re-sells NuView's StorageX, calling it Virtual File Manager (VFM).

But do you really want a global namespace? "Some customers want to see everything as a single tree, and others don't," says NetApp product manager Ravi Parthasarathy. So these products also allow the global namespace to be split into a few smaller ones while preserving the benefits of virtualization.

"Once you have a virtual namespace, you can implement policy-based intelligence," says NuView founder Rahul Mehta. "For example, in the event of a disaster, your remote offices can be redirected to NAS server A in the data center, while your corporate users would be redirected to NAS server B." But make sure your solution supports locking of open files; without locking, two clients at different sites could try to write to the same file at the same time, leading to data corruption.

Comparing NAS products

This was first published in June 2004

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