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I don't think it's clear to me yet, is it faster to read things from a file or from memcached? Why?
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File reads are cached by the OS filesystem, so they are fast memory transfer operations between the kernel and Python. There is no disk I/O involved here. With Redis, you have to pay for the cost of the roundtrips, so it is much slower.
When storing data, Redis stores data as specific data types, whereas Memcached only stores data as strings. Because of this, Redis can change data in place without having to re-upload the entire data value. This reduces network overhead.
Redis uses a single core and shows better performance than Memcached in storing small datasets when measured in terms of cores. Memcached implements a multi-threaded architecture by utilizing multiple cores. Therefore, for storing larger datasets, Memcached can perform better than Redis.
memcached can process over 50 million keys per second on a 48 core machine using only RAM and heavy batching.
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.
(Or man up and invest some money into more memory like these guys :)
For some benchmarks see: Cache Performance Comparison (File, Memcached, Query Cache, APC)
In theory:
Read 1 MB sequentially from memory 250,000 ns Disk seek 10,000,000 ns http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf
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