I tried to post a comment to Kenny Gorman’s post on Tuning for very large shared_buffers article, but it seems to have gone into the spam bucket. I was torn about whether it’s worth a separate post anyway, so this tipped me over the edge.

My question is what happens in the other 3 scenarios that weren’t measured? Namely,

  • Test#4: buffered I/O and 20GB of shared_buffers
  • Test#5: direct I/O and 500MB of shared_buffers
  • Test#6: direct I/O and 2GB of shared_buffers

Without these, I’m unable to form an opinion on the article’s conclusions:

A modest gain can be had when using a very large (comparatively) shared_buffers setting when combining that change with direct I/O. The PostgreSQL cache does scale quite nicely up to at least a 20GB cache size when configured in this manner. I’m also unclear on what the X-axis on the graph represents.

Done! Now Read These: