As file sizes and data sets grow into the terabyte and petabyte range, users are looking for a method for storing, accessing and sharing the files among different hosts.

That's where clustered and storage-area network (SAN) file systems come in.

Vendors have created software and hardware appliances that combine disparate file systems into one file system with one name space. These appliances and software improve users' ability to access data and share the data with others irrespective of the media or host computer on which it sits.

The technology these appliances and software use is known as clustered and SAN file systems. File systems of these types have several advantages over distributed file systems:

• By clustering systems and sharing applications and data, tasks can be performed much more quickly than they could on individual machines because data doesn't need to be copied or replicated from one file system to another.
  
  

• Clustering provides more space for files and file systems.
  
  

• Management is easier because only one file system is being managed, not a file system for each storage device or host computer.
  
  

• Failover is available because one server in the cluster can take over the responsibilities of another if it fails.
  
  

• Users have concurrent access to all files located on the storage devices on their network.
  
  

Working as one

In a cluster, a group of independent nodes or host computers work together as one system. They may share a common storage array or SAN and have a common file system that has one name space. A traditional example is HP's Tru64 cluster file system used in TruCluster systems.

More recent implementations are from Cluster File Systems, Oracle, Red Hat, start-ups Panasas and Spinnaker Networks, and others. Red Hat, which acquired Sistina last year, released its clustered Global File System into the open source; Network Appliance, which acquired Spinnaker Networks, is using its SpinCluster software to improve its grid strategy, which clusters network-attached storage (NAS) and SAN storage. Oracle uses its Cluster File System on the company's Real Application Clusters (Oracle 9i RAC); Cluster File Systems uses its Lustre File System to build high-performance compute clusters.

In the Lustre File System, Panasas and Permabit implementations, individual servers are connected to storage by a metadata server or device, which categorizes each bit of data so it can be found easily.

Mark Seager of Lawrence Livermore National Laboratory and Scott Studham of the Pacific Northwest National Laboratory are using the Lustre File System.

"Before we had a file system on every cluster and had to FTP the data between file systems," says Seager, systems department head. Seager has two 1,000-node clusters in production today.

"Better performance is a key criteria for using a clustered file system," Seager says. "The other issue for us is to not have to replicate the data [to the other cluster] when someone needs it."

Seager's group does scientific simulation and modeling with its clusters. Seager says it's important to be able to read data off the file system and concurrently see the results while the simulation is still going on.