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1. File-system virtualization (aggregation) is one way of virtualizing file access. At a high level, file-system virtualization accumulates individual file systems into a pool that's accessed by clients as a single unit. In other words, clients see a single large namespace without being aware of the underlying file stores. The underlying file store could be a single NAS, or a mesh of various file servers and NAS systems. File-system virtualization products address two main problems: They give users a single virtual file store; and they offer storage management capabilities such as nondisruptive data migration and file-path persistency while files are moved between different physical file stores.

One of the great benefits of file-system virtualization is that it can be deployed in existing environments without having to rip out existing servers and NAS storage. On the downside, file-system aggregation doesn't address the problem of having to manage each file store individually.

2. Clustered file systems are another way of virtualizing file access. Clustered file systems are part of next-generation NAS systems designed to overcome the limitations of traditional scale-up NAS. They're usually composed of block-based storage nodes, typically starting with three nodes and scaling to petabytes of file storage by simply adding additional nodes. The clustered file system glues the nodes together by presenting a single file system with a single global namespace

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to clients. Among the vendors offering NAS systems based on clustered file systems are FalconStor Software Inc.'s HyperFS, Hewlett-Packard (HP) Co.'s StorageWorks X9000 Network Storage Systems, IBM's Scale Out Network Attached Storage (SONAS), Isilon Systems Inc., Oracle Corp.'s Sun Storage 7000 Unified Series, Panasas Inc., Quantum Corp.'s StorNext and Symantec Corp.'s FileStore.

3. Clustered NAS is a third way of virtualizing file access. Clustered NAS architectures share many of the benefits of clustered file-system-based NAS. Instead of running a single file system that spreads across all nodes, clustered NAS systems run complete file systems on each node, aggregating them under a single root and presenting them as a single global namespace to connected clients. In a sense, clustered NAS is a combination of a scale-out, multi-node storage architecture and file-system aggregation. Instead of aggregating file systems of heterogeneous file stores, they aggregate file systems on native storage nodes. The BlueArc Corp. Titan and Mercury series of scale-out NAS systems are prime examples of clustered NAS systems.

4. NAS gateways can also be viewed as file-system virtualization devices. Sitting in front of block-based storage, they provide NFS and CIFS access to the block-based storage they front end. Offered by most NAS vendors, they usually allow bringing third-party, block-based storage into the NAS and, if supported by the NAS vendor, into the global namespace.

NAS systems and gateways based on clustered file system or clustered NAS architectures are next-generation NAS systems and won't integrate with existing legacy file stores; they usually replace them or run in parallel with them. This makes them more difficult to deploy as well as more expensive than file-system virtualization products. However, the benefit of having to manage a single NAS, rather than many small data silos that are simply aggregated by a file-system virtualization product into a single namespace, more often than not justifies the additional effort and cost.

NAS virtualization terminology

Namespace is the organization and presentation of file-system data, such as directory structure and files.

In a non-shared namespace, file-system information is confined to a single physical machine and not shared with others. Traditional scale-up NAS and server-based file stores are examples of products with non-shared namespaces.

Conversely, a shared namespace, also referred to as a global namespace, combines the namespace of multiple physical machines or nodes into a single namespace. It can be implemented by aggregating the namespaces of multiple machines and presenting them as a single federated namespace, as is usually the case in file-system virtualization and clustered NAS products, or it can be achieved via clustered file systems where a single file system spreads across multiple physical nodes.

Scale-up NAS is a file-based storage system that scales by replacing hardware with faster components, such as faster CPUs, more memory and more disks. Its namespace spans one or two nodes clustered for high availability.

Scale-out NAS is a file-based storage system that provides scaling by adding nodes to the cluster. Available in N+1 (single redundant node) or N+M (each node has a redundant node) high-availability configurations, they provide a namespace that spans multiple nodes, allowing access to data throughout all nodes in the namespace.

This was first published in September 2010

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