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"Slab item chunk sizes" suggests they retained memcache's external fragmentation problem (if you mostly have big objects expiring, those slabs of memory won't ever be reused for storing small objects, or vice versa). On memcached with no persistence, you could recover from this by restarting (if you could withstand the availability hit), but what do you do once you're relying on long-lived state?

This is not an issue for fatcache. slab item chunk size introduces internal fragmentation (because items sizes usually don't match "chunk" sizes unless you use slab profile, -z option, to match them up). The kind of external fragmentation you described is due to the eviction strategy memcached uses, which can be avoided by using slab level eviction strategies (Twemcache and latest Memcached both have this support). fatcache does slab level eviction based on write timestamps, which is the equivalent of LRC eviction in Twemcache, and you can read about the mechanism here: https://github.com/twitter/twemcache/wiki/Eviction-Strategie...

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