How can I find which process is causing this IO read (tried iotop already)

I've got a constant 1.4 megabytes/sec of IO read on one server (see iostat output below). What can I do to find which process is the cause ? I already tried iotop and htop, but could not pinpoint the issue with thoses:

Linux 2.6.35.4-x86_64-linode16 (la-machine)     02/16/11    _x86_64_    (4 CPU)avg-cpu:  %user   %nice %system %iowait  %steal   %idle       0.16    0.01    0.11    0.68    0.01   99.03Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtnxvda             42.29         1.39         0.08     568159      30959xvdb              0.45         0.00         0.00        988       1202

Any hint on either using htop/iotop or another tool to help me find out which process is eating the disk ? (This is a server with 0 traffic currently).

Thank you!