Even when I am not doing anything on my Mac, I see a lot of Disk IO: "Data Written per Sec" in Activity Monitor showns continuous writes:

Disk Activity

Is this normal? Why would some process continuously write? I understand there can be some housekeeping activities going on, the "Sleep image" being updated, but does this happen this frequently? (I have not set up Time Machine)

Edit: I have quite some free memory:

Memory Usage

  • Can you share a screenshot of what's running under My Processes in Activity Monitor? – bg2011 Sep 1 '11 at 8:41
  • Now (when I am back home from work) I see that the activity has come down (peak is around 900 KB/s). What was open in the morning is open now: Twitter App, Chrome and text edit (no changes made there. I typed something 4 days ago that I want to remember, but too lazy to save, and to think where to save) – Nivas Sep 1 '11 at 15:40
  • Will post a screenshot when I see a similar scenario. If I dont, will close this Q. – Nivas Sep 1 '11 at 15:41

You can get a better idea what's causing the activity with some of OS X's command-line tools:

sudo iotop -- display of what processes are doing the most I/O, updated every 5 secs.

sudo iosnoop -- shows I/O as it happens, including the process ID and process name doing the I/O, as well as data size, file path, etc

In both cases, you'll need to authenticate as an admin to run them, and use Control-C to exit. They may not tell you precisely why the activity's happening, but they'll at least tell you what processes and files are involved, which usually narrows it down considerably.


In addition to iotop and iosnoop you may use fs_usage.

sudo fs_usage -w -f filesys -e grep  | grep -i ' write '
  • 1
    This is a great answer. I would love it if you edited the highest scoring answer once you have the reputation to add this - it makes that answer much more complete. – bmike Feb 8 '12 at 17:04

A few things: (Written in bullet points to make it concise)

  • The OS itself writes.
  • Open applications write.
  • Browsers write as well, in the case of cookies, and when your RAM is maxed out.
  • Yes, I understand these. But how frequently? I have a lot of free RAM: Now around 5GB (added this detail to question as well) – Nivas Sep 1 '11 at 6:14
  • Every second the computer is on. Computers always write to disk all the time. – JFW Sep 2 '11 at 7:25
  • mega bytes every second? – Nivas Sep 2 '11 at 7:55
  • Depends on what OS you're using, what applications are open, ... – JFW Sep 2 '11 at 15:29

I believe you should be able to use lsof to check what processes are writing to the disk. Here's the man page for it.

  • lsof is a 'snapshot' based utility that shows everything going on within a few seconds time. IF you don't run it at the precise moment that the I/O is taking place, you'll miss it. OS X's utilities (iotop and iosnoop and numerous other users have answered with) are live/refreshing and very customizable utilities that I (IMO/personally) vastly prefer. – Jason Salaz Feb 8 '12 at 16:57

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