How macOS actually works

The Spotlight processes

Updated 2026-07-31 · 3 min read

Seeing an unfamiliar process at the top of Activity Monitor is alarming. These ones are usually fine.

The processes

Process	Job
mds	Metadata server — coordinates indexing
mds_stores	Writes and maintains the index files
mdworker	Short-lived workers that read individual files
mdimport	Imports a specific file's metadata
mdsync	Synchronises index stores

When high CPU is normal

• Immediately after a macOS upgrade
• After connecting a new external drive
• After a large copy, download or restore
• Following sudo mdutil -E

Hours of activity in those cases is expected. It should subside.

When it is not

Sustained activity for more than a day with no obvious trigger usually means indexing is failing and restarting. Common causes: a corrupted index, a drive mounting repeatedly, or a sync client rewriting files continuously.

The fix is generally to rebuild once, and to exclude whatever is churning.

Seeing what it is working on

sudo fs_usage -w -f filesys mdworker

That shows which files the workers are reading, which usually identifies the culprit immediately.

Stopping it

sudo mdutil -i off /

Search stops working while it is off, so use this to diagnose rather than as a solution.

Why a filename index is quieter

Content indexing means opening and parsing files — that is what mdworker is doing when it uses CPU. A names-only index reads directory entries, which is dramatically cheaper, and after the initial scan it only processes changes as they are reported.

Common questions

Is mds_stores a virus?

No. It is part of Spotlight and ships with macOS.

How do I stop mds using CPU?

Identify what it is indexing with fs_usage, exclude that location, and rebuild once.