How to decide whether to enable MySQL query cache using InnoDB How should the decision to enable / disable the MySQL query cache be made, on a server that uses just InnoDB tables. Say for example, if the cache is enabled, then how should the output of:

SHOW STATUS LIKE 'Qcache%';


be interpreted to make the decision? Or what other queries/profiling can be performed to get more information, and then how should those be interpreted? If it's to be enabled, how should its size be determined? I'm on Amazon RDS, using MySQL 5.6. I will end up being around ~250 separate databases totalling ~200gb in space.
I had discussed this in earlier posts


Sep 05, 2012 : Is the overhead of frequent query cache invalidation ever worth it?
Sep 26, 2013 : query cache hit value is not changing in my database


InnoDB internals has a very hands-on approach to the query cache since it micromanages query cache entry invalidation. In the MySQL 4.x days, the query cache was disabled by default for the sake of InnoDB. In MySQL 5.x, it can be a toss-up. The comments from @akuzminsky show local issues center around  InnoDB.

Given the fact you are using Amazon RDS, you find the following challenges


Changing some InnoDB settings can be cumbersome
Some InnoDB settings cannot be changed
Tuning aspects of InnoDB has limits due to RDS


I would take away any guess work and just leave the query cache disabled. If having a bigger buffer pool size (as suggested in @akuzminsky's second comment) you will have to migrate to a larger Server Model ($$$ Cha-Ching $$$). Here are the Buffer Pool sizes for the Server Models

MODEL      max_connections innodb_buffer_pool_size
---------  --------------- -----------------------
t1.micro   34                326107136 (  311M)
m1-small   125              1179648000 ( 1125M,  1.097G)
m1-large   623              5882511360 ( 5610M,  5.479G)
m1-xlarge  1263            11922309120 (11370M, 11.103G)
m2-xlarge  1441            13605273600 (12975M, 12.671G)
m2-2xlarge 2900            27367833600 (26100M, 25.488G)
m2-4xlarge 5816            54892953600 (52350M, 51.123G)