I am currently going through a design phase of a project that will involve the following: 1. Two data centers (approximately 50Km apart) running active/active. 2. The solution will be all Windows based involving two SQL clusters, all are running on HP to include a SAN. All specs are to be determined at this point.
Interested in your thoughts about how the replication would work. Should it be done at the SAN level or SQL level? How good is the HP SAN stuff? Is it rebadge equipment, Compaq/HP? Also, any data from other companies that have successfully implemented active/active data centers solutions would be useful.
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If I were building a configuration like yours, I would try to extend a single SAN between the two locations. It's a much simpler and probably less expensive approach. This way, when it comes to getting data between the sites you're NOT replicating, you're just mirroring data.
50km is well within the specified limits for most SAN vendor implementations.
With this model, you can combine traditional clustering and wide-area data replication with less hardware. You can design clusters so that they failover to the other site, just as cleanly and neatly as if they were on the local site.
Data replication requires an additional step of sending the data across the replication network (regardless of the technology in place). Mirroring is handled much more simply by a software mirroring product. Most hardware-only solutions cannot handle mirroring outside of a single disk array, so you will likely be restricted to a software-based solution.
Unfortunately, I do not have the opportunity to evaluate hardware from different vendors, so it is very difficult for me to offer opinions on specific equipment from different vendors. My advice, as always, is to ask the vendors for customer references, contact the references and then ask them hard questions about the vendor and his products.
Evan L. Marcus
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This was first published in February 2004