Far-flung file serving

Wide area file servers cache documents locally and reduce WAN traffic

WAFS (wide-area file sharing) appliances combine WAN optimization with file-caching in an effort to remove file servers from remote offices. Sometimes called wide-area file-server replacements, they also rely heavily on application-specific acceleration, especially CIFS and MAPI.

The goal of WAFS is to provide for a completely serverless office. In the not so distant future, instead of deploying file and mail servers to remote offices, IT managers will simply ship a preconfigured rack-mount appliance to each remote site, and possibly preloaded with cached data, as a file-server replacement.

One of the things that will separate the real WAFS players from the pretenders is how the appliance handles file-locking. For instance, a user at a remote office opens an Excel spreadsheet over the WAN. The WAFS appliance keeps a copy of the file in its local disk cache. While the user works on the sheet, all saves are done locally on the appliance and not sent over the WAN — greatly improving application response. When the user closes the file, the WAFS appliance then sends the updated document to the file server.

During this editing process, the WAFS box — if it does its job correctly — will have issued a file-lock on the open file on the server, and then released the lock on the final save and close. This gets interesting when the WAN goes down while the file is still open.

The remote user will keep on working on the locally cached copy. But what about the locked file on the file server? What happens if another user opens the same file before the first user gets a chance to save his file? A good WAFS vendor should be able to answer such questions, but there’s no perfect response.

One application that takes great advantage of WAFS advanced caching is distributed backup. Whereas backups from remote offices to the datacenter used to take hours to complete, they now take mere minutes thanks to file-differencing algorithms in the WAFS appliances. So instead of sending the entire file set over the WAN, only the changes are sent, greatly reducing the time needed to complete the backup.

For WAFS to succeed, it has to smoothly marry all WAN acceleration techniques into a comprehensive solution. But where it really shines is the cache-differencing and local-file access. On high-latency links, this alone makes it worth the cost of installation.

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