[CentOS] isolating disk consumer process

I have a weird problem. Sometimes the disk  usage indicator led shows an 
intensive disk access and the server becomes slow. When I try to 
identify what process is consuming  more resources with top command, I 
see that most process are just taking 1% of CPU, or maybe sort peaks of 
15% for  postgresql or amavis.
How can I determine which process is making such intense use of disk?

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