wget http://aspersa.googlecode.com/svn/trunk/iodump
echo 1 > /proc/sys/vm/block_dump
while true; do sleep 1; dmesg -c; done | perl iodump

^C# Caught SIGINT.
TASK PID TOTAL READ WRITE DIRTY DEVICES
jbd2/md1-8 495 75 0 75 0 md1
md1_raid1 467 20 0 20 0 sda2, sdb2
flush-9:1 1246 16 0 16 0 md1

Whats next?

yum install smartmontools
smartctl -a /dev/sda
smartctl -a /dev/sdb

If you find:
197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always – 103

Its time to replace HDD disk.