I run a CentOS 5.7 machine (64bit) with 24GB ram and 4x SAS drives in RAID10 setup. This machine runs nginx/1.0.10, php-fpm & xcache. About a month back the RAM usage of this machine has changed.

About every few hours the 'CACHE' is flushed from the RAM, this happens exactly when the 'Inode table usage' drops. I'm pretty sure these drops are related. (see the 2 attached images).

This server hosts quite a lot of small files (20M all a few KB big). Not many files are deleted (maybe 100 per hour (total size a few MB max)), not enough to account for the huge Inode table drops.

I also have no crons running which could cause these drops.

Sar -r output: http://pastebin.com/C4D0B79i

My question: Why are these huge RAM/Inode usage drops happening? How can I get Nginx/PHP to use all of my servers RAM?