How to check disk activity on a Mac, and which process is reading and writing
iostat shows how hard the SSD is working; fs_usage shows who. Our MacBook Air ran 262 to 421 operations a second at 5 to 8 MB/s at rest. How to read both.
How busy is the disk?
iostat -d -c 3 -w 1| Second on our Mac | KB per transfer | Transfers a second | MB a second |
|---|---|---|---|
| 1 | 19.7 | 421 | 8.1 |
| 2 | 18.3 | 262 | 4.7 |
| 3 | 20.9 | 337 | 6.9 |
Hundreds of small transfers a second at a few MB/s is ordinary background work on a Mac: logging, caches, Spotlight, sync. Sustained hundreds of MB/s with nothing obvious running is not.
Which process is reading and writing?
sudo fs_usage -w -f filesys | head -100Every line names a process and a file. Press Control C to stop. The process that fills the screen is the busy one. Activity Monitor's Disk tab gives totals per process since it started.
See it live
Free Mac's Storage map shows where the space went, and its System tab shows what is running right now. See also app logs and lost disk space overnight.
Related: check your Mac's uptime and How many processes does a Mac run.
Common questions
How do I check disk activity on a Mac?
Activity Monitor's Disk tab, or iostat -d -w 1 in Terminal for live reads and writes.
How do I see which process is writing to disk?
Run sudo fs_usage -w -f filesys in Terminal, or sort Activity Monitor's Disk tab.
What is normal disk activity on a Mac?
Ours ran 262 to 421 transfers a second at 5 to 8 MB/s in ordinary work.