Keep remote offices in sync


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Disk caches
Remote office users may suffer when WAN links slow down, CAD programs open large numbers of files or Microsoft Office applications create multiple temporary files. To ease these pain points, four vendors--Cisco, DiskSites, Riverbed Technologies and Tacit Networks Inc.--now include disk caches in their appliances that keep local copies of centrally stored files. Disk caches at the remote office maintain a limited or complete copy of the central office files needed by the remote office and provide remote users with the following benefits:

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Implementation considerations
WAFS products offer many attractive benefits, but the following questions should be considered before they're implemented:

Do you know what applications remote offices run? Applications use chatty protocols like CIFS and MAPI to communicate with file servers. Although they work fine in remote offices, they're not optimized for WAN connections. Document which applications branch offices use and which files the applications open, create, save and delete.

What happens to existing file servers? When WAFS appliances go into remote offices, remote file servers or NAS appliances go out. Establish how these servers or filers will be managed or disposed of to prevent the remote office from reusing them without your knowledge.

Is there a pre-installation checklist? All vendors promise nearly transparent installations, but ask for a pre-install checklist to ensure it goes as smoothly as possible. The vendor's reaction to this request can tip you off as to how well-prepared they are to do the install.

How will the disk cache be managed? Disk caches increase the performance and availability of files at remote sites, but they also introduce remote management issues. Make sure you verify what RAID levels the disk cache supports, how the appliance reports drive failures and who will manage the appliance when problems occur.

  • Near-LAN speed access to centrally stored files
  • Local access to files should the WAN link drop or the central server go offline
Vendors employ different techniques to raise the hit level of their appliance's disk cache. DiskSites, for example, stipulates that its FileController remote office appliance be installed in the central office first so that it can be loaded with the files the remote site will need. FileController is then sent to the remote office, synchronizes with DiskSites' central office FilePort product and stays in sync thereafter.

Cisco also offers disk caches on its Wide Area Application Engine (WAE) line of products. "The working set of data for branch-office users is typically less than 10% of the total data repository," says Baruch Deutsch, director of product marketing for Cisco's caching division. "Generally, 90% of the branch user file requests can be served locally for the data set in the cache. We've discovered that the amount of storage needed to handle these requests and store these files is often only in the hundreds of megabytes." Despite this statistic, Cisco's remote office WAE disk cache starts at 80GB and tops out at 840GB.

But as the size and number of files on remote disk caches increases, the challenge to maintain and support the availability, consistency and integrity of these files across the enterprise grows. In addition, apps like CAD, e-mail, Office and enterprise resource planning create unique problems such as opening multiple network files, creating temporary files or using chatty network protocols that are acceptable in LAN environments but can choke WAN links. Meeting these issues head-on requires WAFS vendors to fine-tune their products for different mission-critical applications.

Optimizing applications
With applications designed to run in a high-bandwidth LAN environment, a normal application operation like "Open" or "Save" can clog the WAN with network traffic. For instance, to open a single AutoCAD drawing with links to other files results in AutoCAD accessing and opening those other files. Similarly, a "Save" operation from a Microsoft Office application generates 1,000 to 1,500 transactions on the network. While the size of each transaction may be small, a composite of thousands of them slows performance across a WAN link.

Appliance vendors use different techniques to minimize the impact of application file operations on WAN performance. For temporary files created as a scratch file by applications such as AutoCAD or Microsoft Office, DiskSites' FileController and Tacit Networks' IShared use file-aware differencing technology. This configurable option allows these applications to store temporary files on the remote appliances disk cache, but can prevent these files from being transmitted to the central office until the primary file is actually saved.

Cisco's WAE and Riverbed Technology's Steelhead appliances send and receive all files, but minimize network traffic by first breaking up and storing files as file segments. Cisco's WAE transmits only changed file segments across the WAN. Riverbed Technology similarly stores like segments from different files together, but takes the concept of file segments one step further. Its Steelhead stores file segments in lengths of approximately 100 bytes in size; the appliance then recognizes which file segments have the same underlying structure and stores them as one logical segment. This approach led to the discovery that many of the files companies work with are copies of files that already exist. This technique reduces the amount of new data, the data in the remote office disk cache, and the number of changes that need to be transmitted over the WAN.

WAFS products are evolving to address a variety of enterprise-wide concerns. While some of the available WAFS products effectively solve the problem of real-time file consistency, an extended WAN outage can, of course, affect the currency of file synchronization operations. Still, they can help organizations to get much closer to providing central and branch-office users with secured, simultaneous access to files. The introduction of file segmentation and application optimization techniques give remote users the near-LAN speed they require and the centralized file protection and management data centers seek.

This was first published in October 2005

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