How can I monitor hard disk load on Linux?

Is there a good command line utility to monitor hard disk load on linux? Something like top but then monitoring disk activity i.s.o. cpu usage.

More specifically, I suspect that for some (heavy load) servers after several optimizations on various parts of the program(s) that run on it, right now the bottleneck is simply the logging to files on the disk. But I find it very difficult to assess how much traffic the servers can handle.

My ideal tool would be something that prints "You're using 35% of your disk bandwidth right now". Any ideas?

Solution 1:

You can get a pretty good measure of this using the iostat tool.

% iostat -dx /dev/sda 5

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.78    11.03    1.19    2.82    72.98   111.07    45.80     0.13   32.78   1.60   0.64

The disk utilisation is listed in the last column. This is defined as

Percentage of CPU time during which I/O requests were issued to the device (band-width utilization for the device). Device saturation occurs when this value is close to 100%.

Solution 2:

iotop is a version of top that looks at how much IO each process is using. It's in the standard ubuntu repositories; I don't know if it's in RHEL or Fedora, but it should be.

For older kernels (say CentOS 5.x or RHEL 5.x) that do not support iotop, use topio instead (documented here: http://yong321.freeshell.org/freeware/pio.html#linux. It relies on /proc//io for io statistics and provides similar functionality as iotop. See link for further details.