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Virtual memory support is currently being added to Redis so that it won't need to fit everything in RAM either. I'm sure antirez can provide some better info o

• antirez 17y ago
  Hello, VM is already in alpha on Git actually. There is some more work to do, but I don't think in the Reddit use case there is the need of Redis VM: they are using MemcacheDB as a persistent cache, if it's a cache it should match very high performances delivered by memcached and is not required to cache everything. Redis is as fast or faster than memcached (using clients with the same performances) and is persistent, so probably it's a good fit for this problem. Instead of VM Reddit should use Redis EXPIRE I guess, that is, time to live in cached keys so they auto expire. Btw for people that don't know what Redis Virtual Memory is: with VM Redis is able to swap out rarely used keys in disk. This makes a lot of sense when using Redis as a DB. When using Redis as a cache, the way to go is EXPIRE: rarely used things in cache should simply go away and be expired instead of being moved into disk. EDIT: It would be very interesting to know where the Reddit performance problem is, but Redis Sorted Sets are a very good match to create social-news alike sites like HN or Reddit in a scalable distributed way, with a few workers processing recent news to update their score into the sorted set. The home page can be generated with ZREVRANGE without any computation. It's a shame reddit is not sharing how this cache is used.