Elastic caching is just TTLs with an invoice attached
Google Research published a useful production systems result this week: linear elastic caching in Spanner. The headline number is easy to quote. In a production rollout, the policy cut cache memory by 15.5%, raised cache misses by 5.5%, and reduced total cache ownership cost by about 5%.
The part I like is smaller than the headline. They did not rebuild caching around a giant predictor. They changed the question.
Most cache tuning starts with a fixed box: here is 128 GiB, choose the least bad eviction policy. LRU, LFU, GDSF, ARC, CLOCK variants, pick your flavor. Those policies decide what leaves when the box is full.
Linear elastic caching asks something closer to the bill: how much does it cost to keep this page in memory for another unit of time, and how much would it cost to fetch it again if I throw it away?
