From collectd Wiki

The Disk plugin collects performance statistics of hard-disks and, where supported, partitions. While the “octets” and “operations” are quite straight forward, the other two datasets need a little explanation:

• “merged” are the number of operations, that could be merged into other, already queued operations, i. e. one physical disk access served two or more logical operations. Of course, the higher that number, the better.
• “time” is the average time an I/O-operation took to complete. Since this is a little messy to calculate take the actual values with a grain of salt.

[edit] Synopsis

<Plugin "disk">
  Disk "sda"
  Disk "/^hd/"
  IgnoreSelected false
</Plugin>

[edit] Example graphs

[edit] Dependencies

• Linux
  • Either /proc/diskstats (Linux 2.6) or /proc/partitions (Linux 2.4)
• Darwin / Mac OS X
  • I/O Kit framework
• Solaris
  • libkstat(3KSTAT)
• AIX
  • libperfstat(3)