what process is r/w to the disk?

Hello, I have some unknown process reading or writing to device sda. How can I find out process it is? It is really slowing everything else down. Linux 2.6.5-7.111.19-smp 03/07/06 avg-cpu: %user %nice %sys %iowait %idle 10.30 0.01 0.88 4.69 84.12 Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn hda 1.07 10.57 4.38 1473222 611062 sda 57.99 4966.69 223.69 692362762 31183088 sdb 9.98 0.06 213.31 8898 29735896 fd0 0.00 0.00 0.00 4 0 Thank you, James