Monitor disk I/O on Mac

Disk I/O is the most invisible bottleneck on a Mac. A pegged SSD looks like “everything is slow” but Activity Monitor’s CPU and Memory tabs are calm, so people misdiagnose and blame the machine. Knowing how to read disk activity directly saves you an hour every time it happens.

What to use

iostat (system-wide throughput)

The classic Unix tool, present on every Mac.

1. Open Terminal.
2. Run iostat -w 1 disk0.
3. You'll see KB/t (avg KB per transfer), tps (transactions per second), and MB/s (throughput), updated every second.
4. Ctrl-C to stop. Use disk1, disk2 if you have additional volumes.

Good for “is anything hammering the disk right now,” not for “which app.”

Activity Monitor’s Disk tab

1. Open Activity Monitor, click Disk.
2. Sort by Bytes Written or Bytes Read for cumulative-since-start.
3. For live, watch the Data read/sec and Data written/sec in the bottom panel.
4. The graph there shows IO/sec — a steady high line means sustained activity.

fs_usage (per-file detail)

When you really need to know which files:

sudo fs_usage -w -f filesys ProcessName

Spammy but precise — you see every read and write the process performs. Filter by process to keep it manageable. -w widens columns; -f filesys shows filesystem operations.

A menu bar monitor

For ongoing awareness, the menu bar is the right place. Beacon’s Disk readout shows live read/write throughput, and clicking gives you top disk users.

1. Install Beacon and enable Disk in Settings > Menu Bar.
2. Pick combined throughput or separate read/write arrows.
3. Click for the top processes by current I/O.

Reading the numbers

Modern Mac SSDs can hit several GB/s burst, but sustained 200-500 MB/s under real workloads is more typical. If you’re seeing:

• Sustained 500+ MB/s write — something is bulk-writing. Time Machine, a large download, a video render, or a compile to a slow target volume.
• High tps with low MB/s — lots of small operations. Spotlight indexing (mds_stores), database transactions, or a leaky log writer.
• Steady writes when “idle” — Photos analysis, iCloud sync, or a buggy app rotating logs. Use fs_usage to find the file paths.
• Zero I/O but slow Finder — that’s not disk; it’s probably a stalled network volume or APFS snapshot housekeeping.

The general rule: if CPU and RAM look fine but everything feels stuck, look at disk I/O before you look anywhere else.