Weird Slowdown on I/O (4K blocks ?)

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http://serverfault.com – I have two identical servers, running Proxmox 2. 2x AMD 6 cores 2.3Ghz 64 Gigs of RAM DDR3 ECC (8*8) ASUS KCMA-D8 2x Seagate 2TB - (st2000dm001) One server is running perfectly fine. The other one is not. As long as something around I/O is involved, the CPU load jumps (only of iowait). Formatting the hard drive takes a LOT of time. And when running the OS it feels like it is slower. I thought about defective hardware, so I switched everything : PSU, RAM (all of them), SATA cables, SATA hotswap backplane, motherboard, hard-drives, etc... Still I have one server running fast, the other one be (HowTos)