In the Activity monitor, under the tab Disk,

enter image description here

at the bottom center, there is a small graph which can be set to either IO or DATA

enter image description here

enter image description here

The two graphs look somewhat similar. I understand that I/O very likely stands for Input / Output.

What is the difference between IO and DATA?

I am on macOS 10.11.3 (El Capitan)


Apple provides an explanation on their Support site:

The Disk pane shows the amount of data that each process has read from your disk and written to your disk. It also shows "reads in" and "writes out" (IO), which is the number of times that your Mac accesses the disk to read and write data.

  • Disk I/O is the number of reads/writes that are done (how many)
  • Data is the aggregate amount of data transfered (how much)
  • Thank you! So if I do cp sourceFile DestFile, I suppose it is going to access the disk twice (once to read and once to write). It is not going to access the disk twice for every line or twice for every chunk of max n bytes (whatever n is), right? – Remi.b Aug 18 '17 at 19:05
  • 1
    At minimum it's going to access the disk twice. You have to account for services like spotlight indexing, preview generation, directory access, caching, etc. If you want to see all the currently open files by whom issue the command lsof in Terminal. For full details on how it works, type man lsof – Allan Aug 18 '17 at 19:09

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .