There are potentially multiple caches between the web browser and the application server: the client’s browser cache, intermediary caches, content delivery networks (CDNs), and the load balancer or reverse proxy sitting in front of the application servers.
Caching, even at the reverse proxy/load balancer level, can greatly improve performance.
Nor does this appear on any of my systems here, they all have correct permissions.
To clear a non-Drupal cache, please refer to the procedures for those caches. Do not forget to set this back to FALSE afterwards.
Use the following code to clear specific cache type from within a module.
Note that there are sometimes caches other than the Drupal cache that can affect website performance.
For example, some hosting services use a cache called "Varnish" to improve performance.
Clearing the caches can also be useful when installing new modules or themes, and as a first step in troubleshooting.
During development it is necessary to clear the cache very frequently. Sites might slow down for a bit afterwards while the cache fills back up.
We all know that the performance of applications and web sites is a critical factor in their success.
The process of making your application or web site perform better, however, is not always clear.
After discussing with Kris Moore he thinks we should fix this before we release 11. (I tried that.) Mostly this seems to be okay, but making sure the update cache directory isn't so restrictive would be best.