Eweek has a long interview with Marty Abbott, SVP of technology at eBay, who walks us through their online operations. Among other things, he explains how the backend databases evolved over the last five years:

"We went from one huge back-end system and four or five very large search databases. Search used to update in 6 to 12 hours from the time frame in which someone would place a bid or an item for sale. Today, updates are usually less than 90 seconds. The front end in October ’99 was a two-tiered system with [Microsoft Corp.] IIS [Internet Information Services] and ISAPI [Internet Server API]. The front ends were about 60 [Windows] NT servers. Fast-forward to today. We have 200 back-end databases, all of them in the 6- to 12-processor range, as opposed to having tens of processors before. Not all those are necessary to run the site. We have that many for disaster recovery purposes and for data replication."