Redis - Can data size be greater than memory size?

Redis loads everything into RAM. All the data is written to disk, but will only be read for things like restarting the server or making a backup.

There are a couple of ways you can use it with less RAM than data though. You can set it up in combination with MySQL or another disk based store to work much like memcached - you manage cache misses and persistence manually.

Redis has a VM mode where all keys must fit in RAM but infrequently accessed data can be on disk. However, I'm not sure if this is in the stable builds yet.