From LQWiki

iostat allows you to view which block deVices are under heavy I/O load. iostat's output looks like this:

tweedleburg:~ # iostat
Linux 2.6.25.18-0.2-default (tweedleburg)       12/12/08

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.52    0.00    0.54    0.89    0.00   96.05

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda               8.75        76.23       322.62   40950470  173309808
sda1              8.68        76.23       322.62   40950126  173309808
sdb               0.00         0.01         0.00       4184        720
sdb1              0.00         0.00         0.00        779          0
sdb2              0.00         0.01         0.00       3029        720
sdc               0.00         0.00         0.00       2638         48
sdc1              0.00         0.00         0.00       1142         48
sde               5.94        48.31       708.14   25952348  380412440
sde1              5.94        48.31       708.14   25951388  380412440