In running a VPS with a few Drupal sites, I ran into the baffling instance of having an underutilized query cache, but a relatively high prune to insert ratio. Initially set up on a 360MiB Linode instance, the query cache was at a size of 20MiB with approximately 4MiB (20%) free. After Linode improved their RAM allocations and the server had 512MiB of RAM available, I correspondingly increased the query cache to 32MiB but a larger portion remained unutilized with 10MiB (30%) now free. I don't have historical numbers on the number of prunes, but the value seemed to remain similarly high.
