Is there a big performance hit with using file_store for storing the cache as opposed to mem_cache_store?

Question

I don't think I'm at the point where I need to go through and get memcached setup for my Rails app, but I would like to do some simple caching on a few things.

Is using file_store for as the config.cache_store setting sufficient enough? Or will having to access files for data over and over kill the benefit of caching in the first place from a server load stand point?

Or maybe I'm not really understanding the difference between file_store and mem_cache_store...

Answer 1

The file_store will cache stuff in files in a filesystem.

If that filesystem is LOCAL to your web server, then clearly, it will only be relevant to that particular web server, therefore you'll lose cache hit rate when a cached entity exists on one server but not another.

How many web servers do you have? 2? 10? 100?

The file_store for caching does not scale properly and will reduce your hit rate over a shared store (for example the memcached one).

The purpose of using memcached is so that a pool of web servers can access a single cache (even though it can be split on multiple physical servers). Using the file_store will not (unless it's a network filesystem, and that will almost certainly be fraught with its own problems).

Always measure the cache hit rate on any cache; if you're not getting a high hit % then it's usually not worth it. Be sure to trend it over time in your monitoring.

Answer 2

I don't think I'm at the point where I need to go through and get memcached setup for my Rails app, but I would like to do some simple caching on a few things

Then use your existing database to store your cached items. (You are using a database, right?)

memcached is only a fast-but-dumb database. If you don't need the ‘fast’ part(*) then don't introduce the extra complexity, inconsistency and overhead of having a separate cache layer.

memcache with file_store is a dumb-but-not-even-fast database, and thus of little use to anyone except for compatibility/testing.

(*: and really, most sites don't. Memcache should be a last resort when you can't optimise your schema, denormalise it for common queries or pre-calculate complex operations any further. It's not something the average web application should be considering as essential for scalability.)