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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

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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

4 Answers

up vote 7 down vote accepted

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.

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up vote 5 down vote

In addition to iotop and iosnoop you may use fs_usage.

sudo fs_usage -w -f filesys -e grep  | grep -i ' write '
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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
up vote 1 down vote

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.
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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
up vote 0 down vote

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.

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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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