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Roll your own NAS cluster
Sometimes it's difficult to find a good out-of-the-box network-attached

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storage (NAS) clustering solution. That's the situation that Todd Moore from Dynamic Graphics Group found himself in. "We needed a cluster of NAS servers that could share with CIFS, NFS and HTTP, provide both availability and scalability and would integrate with our legacy applications," says Moore. He wasn't satisfied with available out-of-the-box products, so his team rolled their own, leveraging PolyServe's Matrix Server, a SAN file system.

Dynamic Graphics' solution consists of a four-node Linux cluster sharing 11TB of StorageTek storage area network (SAN) storage with Matrix Server. Two nodes act as NAS servers using open-source Samba software, while another serves a legacy application. The last is a dedicated backup and administration host. Matrix Server allows applications on all four hosts to see the same storage at the same time--at SAN speeds--and share it with a variety of protocols.

At first, Moore had reservations about the system. He was skeptical about mounting the same LUNs on four separate servers at the same time, and also about the difficulty of implementing the PolyServe software. In fact, neither PolyServe nor StorageTek had ever implemented the architecture Dynamic Graphics wanted. But after a customized demonstration Moore felt like "PolyServe's biggest customer." Technicians from both suppliers kept an open mind and worked together to make everything come together.

"It really does work," says Moore. "Thinking outside the box allowed us to have a system that does exactly what we wanted it to do. We couldn't have had that otherwise."
There are two main ways to scale network-attached storage (NAS) systems: You can use a NAS aggregation product such as Network Appliance Inc.'s (NetApp) SpinServer that creates a virtual file system encompassing multiple NAS servers; or use a storage area network (SAN) file system, which allows multiple SAN-attached hosts to access the same file system simultaneously. While these solutions appear similar, they are different both technically and in the benefits they deliver.

Many businesses have long used NAS arrays such as the NetApp Filer and EMC Corp. Celerra. But the capacity and performance limits of single NAS systems have pushed users to deploy multiple servers, either integrated NAS arrays with dedicated storage or NAS heads that sit in front of SAN storage. This has led to an undesirable proliferation of network-accessible file systems, or namespaces, within an environment. NAS aggregation allows multiple NAS servers to be combined and presented to clients as a single large file system, hiding the physical NAS devices behind it. There are numerous benefits to this concept, including having a single namespace with nearly unlimited capacity, as well as redundancy and performance.

SAN file systems, on the other hand, allow multiple SAN-attached servers to read and write to the same file system on the same LUNs. In other words, every server "sees" the storage and the file system on top of it as its own, and reads and writes are arbitrated by the SAN file system software, functioning as a traffic cop for storage blocks. The benefits are different, mainly in terms of performance and scalability of individual applications. Most SAN file systems serve a clustered application like Oracle 9i RAC, allowing all cluster members to access the same data at the same time over high-performance Fibre Channel (FC).

Three key points differentiate NAS aggregation from SAN file systems.

  1. The number of hosts: Most common SAN file systems are intended to allow just a few large servers--usually 16 or fewer--to share access to data. NAS aggregation devices, by contrast, support hundreds of hosts.
  2. Connectivity: NAS aggregators leverage file sharing protocols such as CIFS and NFS over Ethernet and TCP/IP, while SAN file systems require block-level access over FC.
  3. Implementation: NAS aggregation is usually performed by special network devices, while SAN file systems are usually implemented in software on the clients.
NAS proliferation
The proliferation of NAS systems leads to the proliferation of mount points. In other words, if each file server has four shares, and each NAS filer has four more and there is four of each type, then there are 32 independent mountable entities on the network. The problem grows if you need to add another NAS server because you would have to make sure everyone was mounting the new share in addition to the old one. Many Windows environments have run out of drive letters because Windows only supports 24 shares. Although Windows 2000 and above allows shares to be mounted without drive letters, users are so accustomed to referring to "the N drive" that they start getting confused by other mounting methods.

Unix environments have turned to the automount daemon, a network service that allows an administrator to push out a map of network shares and their mount points. This lets Unix users adapt to an ever-changing landscape of network shares, but puts even more burden on administrators to keep everything in order.

Another problem related to NAS is that systems are so easy to set up that users think they are also easy to manage. Many businesses have NAS boxes that are owned and managed by user departments, rather than central IT. These boxes tend to be huge, unmanaged repositories of random files, seldom backed up and constantly out of space.

This was first published in June 2004

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