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High disk I/O when cache is used?

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http://serverfault.com – 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 (HowTos)