A few days ago I noticed a disk I/O wait and disk activity drop (which was great). Then I also notice that my cache was full(\*) and fragmented. Then I flushed cache. After that, disk latency and disk activity jumped to the previous level (which was bad).

IOtop shows that [jbd2/sda2-8] and [flush-8:00] are always on top of disk usage. This is a Dell R210, hardware RAID 1 (H200) with a lot of free memory (16 GB in total, of which about 8 GB are buffer/cache).

(\*) The cache is APC opcode cache for PHP, which reduces disk access for PHP script execution. Cache was full and fragmented because it included files from development instance. When I noticed that, I filtered them out.

The question is: why disk I/O increases when theorically it should decrease? Below are some graphs from munin. Cache was full from Feb 6 to 8.

![enter image description here](https://i.stack.imgur.com/ZVcTQ.jpg)
![enter image description here](https://i.stack.imgur.com/VKlPL.jpg)
![APC cache is currently ok.](https://i.stack.imgur.com/wZ9aa.jpg)

Change after I commented out apc.mmap\_file\_mask as told by @cyberx86

![enter image description here](https://i.stack.imgur.com/5QVaM.png)
![enter image description here](https://i.stack.imgur.com/Hs6Y5.png)

And after a few days <https://serverfault.com/a/362152/88934>