From the article... Evernote shard their 9.5 million total users across 90 shards, where each shard is represented by two of the four or four drives in a mirro

From the article... Evernote shard their 9.5 million total users across 90 shards, where each shard is represented by two of the four or four drives in a mirrored RAID. Also, they are using Ehcache as their object-caching layer, which we can assume is their alternative to memcached. The choice between the two probably coming down to sticking to their Java stack. I'm sure we'll here more in the promise of future articles focusing more on individual subsystems, as the article is pretty shallow in connecting the dots.