Which is faster/better for caching, File System or Memcached?

Question

I don't think it's clear to me yet, is it faster to read things from a file or from memcached? Why?

Answer 1

You're being awefully vauge on the details. And I believe the answer your looking for depends on the situtation. To my knowledge very few things tend to be better than the other all the time.

Obviously it woudln't be faster to read things of the file system (assuming that it's a harddrive). Even a SDD will be noticably slower than in-memory reads. And the reason for that is that HDD/FileSystem is built for capacity not speed, while DDR memory is particulary fast for that reason.

Good caching means to keep frequently accessed parts in memory and the not so frequently accessed things on disk (persistent storage). That way the normal case would be vastly improved by your caching implementation. That's your goal. Make sure you have a good understanding of your ideal caching policy. That will require extensive benchmarking and testing.

Answer 2

Memcached is faster, but the memory is limited. HDD is huge, but I/O is slow compared to memory. You should put the hottest things to memcached, and all the others can go to cache files.

for benchmarks see: Cache Performance Comparison (File, Memcached, Query Cache, APC)

Answer 3

There are quite a few different aspects that might favour one or the other:

• Do you need/want to share this data between multiple servers? The filesystem is local, memcached is accessed over a network.
• How large are the items your caching? The filesystem is likely to be better for large objects.
• How many memcached requests might be there per page? TCP connections and tear-downs might take up more time than a simple filesystem stat() on the local machine.

I would suggest you look at your use case and do some profiling of both approaches. If you can get away with using the filesystem then I would. Adding in memcached adds in another layer of complexity and potential points of failure (memcached client/server).

For what it's worth the other comments about disk vs memory performance might well be academic as if the filesystem data is being accessed regularly then it'll likely be sitting in OS or disk cache memory anyway.