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ketralnis will likely remember this better, but by my recollection we switched to cassandra after we hit the limit of what we could do with memcachedb:

http://blog.reddit.com/2010/01/why-did-we-take-reddit-down-f...

We were already using memcachedb as a persistent keystore, "caching" precomputed listings and comment threads. When it started becoming more trouble than it was worth to maintain, we decided to try cassandra as a beefier replacement.

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It was capping memcachedb's performance that was the last straw, but it became pretty clear that we were going to similarly cap the performance of any disc that we could afford to put in a postgres write master and that we needed a solution that eliminated that bottleneck, as well as it as a single point of failure. Cassandra (or more generally the dynamo model) did that nicely.

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