How can I monitor hard disk load on Linux?

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%.

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.

As suggested by sastanin in comments, you can process directly values given in /sys/block/sda/stat or /proc/diskstats. This may be helpful when none of the other mentioned tools are available and you can't install them easily.

For instance (IIUC) something like this (in bash) should give the number of reads per second :

OLD=`awk '{print $1}' /sys/block/sda/stat` # First field is number of read I/Os processed
DT=1
for ii in `seq 1 10`
do
    sleep $DT
    NEW=`awk '{print $1}' /sys/block/sda/stat`
    echo $((($NEW-$OLD)/$DT))
    OLD=$NEW
done

See https://www.kernel.org/doc/Documentation/iostats.txt and https://www.kernel.org/doc/Documentation/block/stat.txt for documentation.

You should take a look at atop, which combines the power of iotop/top/iftop, all in one place, and highlights the critical parts on your system.

I would recommend taking a look at the nmon tool. It will show you live load on a number of system parameters as well as recording data to a file for later perusal. It's a free tool available here:

to find out what your total bandwidth is you might use hdparm -T /dev/sda to test the buffer cache (fileio) performance hdparm -t /dev/sda to test the device read performance

examples: my laptop gets 82MB/sec from the SATA disk and 2GB/sec from the cache. My dekstop gets 12GB/sec from the cache and 500MB/sec from the HW RAID array. I suspect those last numbers could be double on server class hardware.

set readahead higher than 256, 4096 works best for me

for i in 128 256 512 1024 2048 4096 8192 16384 32768  
do  
hdparm --setra $i  
  for j in 1 2 4 8 16 32  
  do  
  time dd if=/dev/sda of=/dev/null bs="$j"k  count=<fixthis> 1GB / blocksize  
  done  
done  

times reading 1GB at different block sizes and different read-aheads

The standard tool for showing hard disk load is iostat.

It won't tell you how much %age disk bandwidth you're using, since it doesn't know how much bandwidth your disk has. In any case, your disk only has the manufacturer's quoted figure for large transfers of contiguous data.

I think RRDtool should do what you want here it uses a daemon to dump system data and then allows you to process it however you like. I have often used it to produce graphs etc. to measure system load.

htop is a nice tool to check process information. You can add or remove columns and among them are read and write speeds of processes.