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I have a Rails app running on AWS, and in AWS I have a Memcached ElastiCache node of type cache.m3.medium (3GB memory, moderate network performance).

When I look at the Unused Memory graph I see that I do not exceed 335,000,000 bytes of memory (about 0.3GB), which led me to believe I am under-utilizing my cache. However, here AWS says:

Because Memcached overhead uses memory in addition to that used by data, UnusedMemory should not be considered to be the amount of memory available for additional data. You may experience evictions even though you still have some unused memory.

From a Rails console session on the server, when I run Rails.cache.stats I get the following values:

  • bytes: 2552596313 (2.55gb)
  • limit_maxbytes: 2883584000 (2.88gb)

So there I seem much closer to the limit. I tend to believe those stats more than AWS, especially given their definition of "unused memory."

Am I reading things correctly here?

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