Memcached is terrific! But not for every situation...

• You have objects larger than 1MB.
• Memcached is not for large media and streaming huge blobs.
• Consider other solutions like: http://www.danga.com/mogilefs

• You have keys larger than 250 chars.
• If so, perhaps you're doing something wrong?
• And, see this mailing list conversation on key size for suggestions.

• Your hosting provider won't let you run memcached.
• If you're on a low-end virtual private server (a slice of a machine), virtualization tech like vmware or xen might not be a great place to run memcached. Memcached really wants to take over and control a hunk of memory -- if that memory gets swapped out by the OS or hypervisor, performance goes away. Using virtualization, though, just to ease deployment across dedicated boxes is fine.

• You're running in an insecure environment.
• Remember, anyone can just telnet to any memcached server. If you're on a shared system, watch out!

• You want persistence. Or, a database.
• If you really just wish that memcached had a SQL interface, then you probably need to rethink your understanding of caching and memcached. See the blog post below by dormando for more thoughts on this.

• Links:
• 2008/08
• Memcached versus more rdbms sharding at highscalability.com
• Should you use memcached? Should you just shard mysql more? by dormando