For many years, the enterprise has been able to replicate storage volumes through the storage infrastructure, either locally or remotely, as a means of disaster recovery and business continuance or for other purposes, such as data mining and application testing. Hardware vendors such as EMC (Symmetrix Remote Data Facility--SRDF) and HP (Data Replication Manager--DRM) have provided this capability, but it has lacked solid support in Windows and key applications such as Microsoft Exchange Server and Microsoft SQL Server. With the release of Windows Server 2003 a year ago, I thought data replication would become a more mainstream technology in Windows data centers. That hasn't happened yet, however.
There are probably several reasons why storage replication hasn't made it to the mainstream in the Windows world. The first reason is one of basic need. Many Windows services and the data entrusted to them just don't create a business requirement that justifies the investment in technologies such as storage replication. After all, storage replication isn't cheap, requiring extra infrastructure and disks. In addition, most hardware vendors charge a premium for this added functionality, and that premium might result in a negative Return on Investment (ROI) after business needs are weighed.
Another reasonable explanation for the lack of adoption of storage replication solutions in the Windows space is OS and application support. Although Microsoft provides Windows Hardware Qualification Labs (WHQL) certification for hardware that provides data replication, from a core OS perspective, Windows offers no real support for data replication technologies. To provide such support, Microsoft would need to invest heavily in specific I/O primitives that would understand replication. In addition, if Microsoft chose to support storage replication natively in the OS, the additional testing that would be required would further extend already long ship cycles. And all that is just for the OS. When you consider the applications that run on top of Windows, such as Exchange Server and SQL Server, the story becomes even more complex.
Microsoft is making headway in this area and has done a significant amount of work in Windows 2003 to support geoclustering solutions (clusters that are geographically dispersed and combined with storage replication technology) that include features such as majority node set clustering, which is part of Microsoft Cluster Server. The applications have a lot of work to do, however. Only SQL Server has made strides in this area. Microsoft's other flagship .NET Enterprise Server product--Exchange Server--has no comprehension of a replicated storage environment. On the bright side, I think Microsoft is hearing customers' requirements and will invest more in this area in the future. Until that happens, you're at the mercy of storage solution vendors to help you make storage replication work.
A final reason for the slow adoption of storage replication technology might be the manageability and complexity issues of storage replication solutions. After you start down the path to storage replication, you depart from traditional storage management as you know it. You'll be dealing with proprietary technologies that let a storage volume be replicated from one storage enclosure to another. Replication might be local or remote and subject to transport-media latencies that you don't experience with traditional storage infrastructures. All this makes storage replication technology very complex and more difficult to manage--not to mention more expensive.
In the end, it will be customer demand that solves all these problems. As mission-critical enterprise data drives requirements for better business continuance technologies, storage vendors will respond and the competition will drive storage replication toward commoditization and the mainstream. Then, OSs and application vendors such as Microsoft will invest more in supporting storage replication out of the box.
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