Salvatore Sanfilippo about why Redis 2.2.11 is a recommended upgrade:
[…] since Redis 2.2.7 we introduced a new kind of dictionary (hash table) iterator in order to reduce copy-on-write while a child is saving. I severely underestimated the complexity of porting all the code to the new iterator without bugs, so Redis experienced a number of bugs starting from 2.2.7 due to misuses of the new iterator API in different spots of the code.
The latest bug I found of this kind, hopefully the last, is particularly critical as it is in the persistence code.
Original title and link: Redis 2.2.11: Recommended Upgraded Fixing Critical Bug With Dictionary Iterators (NoSQL database©myNoSQL)