memcached is an in-memory caching layer that is used by many websites to improve performance. Brad Fitzpatrick originally wrote it to help LiveJournal scale.

• Facebook uses and contributes to memcached. As of April, 2008, Facebook was using 805 memcached servers. From the same source, Wikipedia was using 79, Flickr 14, and Fotolog 40.

• Ted Dziuba of uncov fame may publicly mock you if you write a blog post about using memcached to scale your site.
• memcached has a 1MB max object size. If you need to store bigger objects, split them up, store the supplemental object count in the first object and have a naming policy for the supplemental keys. (e.g. “foo” says it has 5 parts → get “foo-2”, “foo-3”, “foo-4” and “foo-5” too.)
• set a chunk_size that's right for your usage

• use the multi-key get whenever you can, it's faster

• Presentation describing memcached internals. For a condensed version of the slides, check out this summary.