linux - How can I monitor disk io?

To see current utilization broken out by device, you can use the iostat command, also from the sysstat package:

$ iostat 1 60

Linux 2.6.32-431.el6.x86_64 (test  05/20/2016  _x86_64_ (12 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           3.08    0.00    0.47    1.55    0.00   94.90

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sdb               0.75         0.02        12.93     520330  402540384
sdc               0.29        32.20       139.68 1002387986 4348777784
sda               1.83        20.28        36.29  631318640 1129811184
sdd              79.17     13919.56      1392.78 433372035354 43362984448