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What’s causing all the disk activity on OS X?

Does anyone know of a program or command for OS X that will show disk writes and what service or program is attached to them?

I have a Mac Pro with a drive that's down to no space available; I delete a couple files, free 2GB and then watch as the space goes down again; a few hours later it's down to 0 again.

There are no obvious programs running, and no software updates have been downloaded in the past couple weeks (at least not in /Library/Updates). Any easy way to find the culprit?

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marked as duplicate by Ƭᴇcʜιᴇ007, Doug Harris, Renan, slhck Oct 11 '12 at 17:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2 Answers 2

Try running the iotop(1m) command from the Terminal. This will give you a list of processes sorted by disk I/O usage. The "BYTES" field tells you how many bytes were part of the I/O operation (whether read or write).

This was also covered in a previous Super User question, here:

What's causing all the disk activity on OS X?

EDIT: Regarding bitesize.d mentioned by someone else, I don't think its as useful because its purpose is specifically to track the size of individual I/O events per process. It's a debugging/investigative tool to see which processes are writing larger or smaller chunks of I/O at a time, and therefore which processes are making efficient use of the disk, not to see which processes are most actively writing the disk.

It can be used for the latter, but requires some extra steps. You have to tally up the histogram bins for the larger I/O sizes (value field) by multiplying the bin length by the size of the I/O operation. If you have a large number of processes performing I/O, this can get tedious really fast.

Another disadvantage is that bitesize.d has to gather data before it displays results since it needs to compute the histograms. Therefore, for "online" analysis of disk usage, I recommend iotop over bitesize.d.

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In Terminal run: bitesize.d. It will print a list of processes that are using the disk.

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