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Comment by dsr_ | Hacker News Reader

This is correct. Any filesystem using the kernel's filesystem cache will do this, too.

For a long running, non-idle system, a good rule of thumb is that all RAM not being actively used is being used by evictable caching.

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A colleague who was used to other UNIXes was transitioning to Linux for a database. He saw in free that used was more at more than 90%, so he added more ram. But to his surprise it was still using 90%! He kept adding ram. I told him that he had to subtract the buffer and cached values (this was before free had the Available column).

Before the Available column there was the -/+ buffers/cache line that provided the same information. Maybe it was too confusing.

             total      used      free   shared buffers    cached
      Mem: 12286456  11715372    571084        0   81912   6545228
  -/+ buffers/cache:  5088232   7198224
     Swap: 24571408     54528  24516880

https://www.linuxatemyram.com/ is one of my favorite single-serving sites.

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