Why are mds_stores and mdworker using so much CPU on my Mac?

What mds, mds_stores and mdworker are

They’re all part of macOS, and Apple’s manual pages describe them (type man mds or man mdworker in Terminal):

• mds is the metadata server. It serves everything that searches file information, Spotlight included.
• mdworker is the worker mds uses to scan and index files as a disk is connected or a file changes. On current macOS it often appears as mdworker_shared, and several copies at once is normal.
• mds_stores looks after the index files themselves, the metadata stores that Apple’s mdutil tool manages.

Apple says the index stays on your device and isn’t shared with Apple. Spotlight, Siri and the search in apps like Mail use it, which is why turning it off is rarely the answer.

The one-minute check

1. Press Command-Space. While indexing runs, Apple says Spotlight shows a progress indicator in its window. If you see one, it’s indexing, and the fix is time.
2. Open Activity Monitor (it’s in Applications, Utilities), click CPU and type md in the search field to see mds, mds_stores and the mdworker processes together.
3. Click Disk. Indexing reads a lot, so these processes near the top of the list of data read is another sign it’s working through your files.
4. For each disk’s status, type mdutil -s / in Terminal. “Indexing enabled.” is normal; repeat with the path of an external drive, like /Volumes/Backup.

Common causes, and what to do

A macOS update or upgrade

Apple lists software updates and upgrades as a time Spotlight indexes, and a full index can take hours or days. This is the most common reason for high mds_stores CPU.

What to do: let it finish. Apple says indexing finishes sooner when the device is idle, plugged in and connected to Wi-Fi or Ethernet, so leave it on power overnight with the lid open or the display just asleep.

A new Mac, Migration Assistant or a restore

Apple says Spotlight indexes at initial setup and whenever data changes. Moving everything from an old Mac or a backup means indexing every file at once, so the first day or two on a new Mac runs busier and warmer.

What to do: the same: plugged in, idle, online. It’s a one-time cost.

An external drive

Apple’s mdworker manual says files are indexed as a volume is mounted, so a big drive you plug in for the first time gets indexed from scratch.

What to do: if you never search that drive, exclude it: System Settings, Spotlight, Search Privacy (lower right), click the Add button and choose the drive. Apple notes one exception: a Time Machine backup disk keeps being indexed even if you add it, because Time Machine needs the index.

A folder that changes all the time

Because Spotlight reindexes whenever data changes, a folder that’s rewritten constantly keeps it busy for good: build output, virtual machine disks, a download or sync folder with thousands of files, app caches you’ve moved into Documents.

What to do: add that folder to Search Privacy the same way. Apple warns that excluding some folders can stop notifications about some app updates, and excluding the whole internal disk stops them altogether, so exclude the busy folder, not the disk.

Indexing that never finishes

If it’s still busy after a couple of days on power, or Spotlight results are missing things, the index may be damaged. Apple’s fix is to rebuild it: in Search Privacy, add the disk or folder, wait a few seconds, remove it again and click Done. Spotlight then reindexes it, which starts the clock again, so do it once.

Not enough free disk space

Apple’s mdutil manual notes that indexing may be delayed by low disk space, so a nearly full disk can keep it from finishing. See Why is my Mac storage full?

Why indexing makes a Mac hot and slow

Indexing keeps the processor and the disk busy for hours, so a MacBook can get warm and the fans can spin up, and a fanless MacBook Air slows down instead. If the heat builds, macOS steps in: kernel_task rises in the CPU list as it holds the busy processes back. See What is kernel_task? and Why is my Mac so hot?

What not to do

• Don’t force quit mds_stores or mdworker. macOS starts them again and indexing carries on, so it only makes it take longer.
• Don’t switch Spotlight off for the whole Mac. Search in Finder, Mail and other apps depends on the index, and Apple says excluding the internal disk stops app update notifications. Exclude busy folders instead.
• Don’t rebuild the index again and again. Each rebuild starts the hours of indexing over.

Keep an eye on it from the menu bar

Activity Monitor names mds_stores once you open it. Multimeter keeps the totals in the menu bar, so you can tell whether indexing is still running without opening anything:

• CPU: total use across all cores, every 3 seconds on power. CPU that stays up while you aren’t doing anything, the evening after an update, is indexing at work; when it falls back to idle, it’s done. Yellow above 70%, red above 90% by default.
• Thermal pressure: Nominal, Fair, Serious or Critical. Long indexing on a warm laptop can push it to Fair; Serious or Critical means macOS is slowing things down to cool off. It’s a state, not degrees.
• Disk space: how full the startup disk is, since low space can hold indexing up.

Multimeter shows totals, not per-process lists, so it can’t tell you that Spotlight is the one using the CPU; Activity Monitor does that. Five live meters use about 3% CPU, and they pause while the displays sleep.