Redis for both data store (jobs) and fragment cache in Rails

Solution 1:

Same redis can be used for both ephemeral data(for fragmant caching) and persistent data given you have configured your redis to write that data to disk which i assume is there as the current redis is being used for persistent data. The evaluation of a separate redis instance can be done based on the volume of use after introduction of fragmant cache. If fragmant cache will significantly increase the load on redis then use a different redis which need not write to the disk itself.

From a configuration and maintenance point of view, both memcache and redis configuration, clustering are equally straightforward when you use it on your own machine or go with existing solution like elastic-cache. Given you are already using redis i would say expanding techstack always adds a little maintenance overhead given you will have additional stack to tweak and tune for better performance.