mds process on Mac: what it is, why it is high, what to do

What mds does

Spotlight is the search behind ⌘Space, but also behind Finder's search box, Mail's search, the Open dialog in every app, and features like Smart Folders and tags. For that to work instantly, macOS keeps an index of every file's name, contents and metadata, and keeps it current as files change. The process in charge of that index is mds. As OSXDaily put it back in 2010, when the name first started puzzling people, "mds stands for metadata server and the mds process is part of Spotlight."

It does not do the heavy lifting itself. Three processes share the work:

• mds watches the file system for changes, keeps the list of what needs indexing, schedules the work, and serves search queries from the finished index.
• mdworker (several copies, plus mdworker_shared) opens each file and extracts the text and metadata. These are the ones that spike per file.
• mds_stores writes what the workers found into the index files on each volume.

Howard Oakley's description of a re-index is the one to keep in mind: the busy processes "should include mds_stores, mdworker (often multiple copies) and mds itself." Three names, one job. If you see all three near the top of Activity Monitor, Spotlight is indexing, and that is the whole story.

A fourth cousin, corespotlightd, indexes content inside apps (mail, notes, messages) rather than files, and has its own page.

Why mds is using CPU right now

1. A macOS update. Major versions change the index format and Spotlight rebuilds it. The most common reason by far, and the one people forget because the update was yesterday.
2. A lot of new files. A photo archive copied in, a backup restored, a large download, a code checkout with a hundred thousand small files. Each one is scanned once.
3. A drive it has not seen. External SSDs, camera cards, Time Machine disks and network shares all get indexed on first connection, and re-indexed if their index was damaged. The 2007 MacRumors thread that still ranks for this described exactly that: "During such (re)indexing, the mdworker goes mad and takes 80-90% of CPU, fans go wild."
4. Something writing constantly. A sync client re-downloading files, a mail account fetching a large archive, a virtual machine or Docker image changing all day. mds sees every write as a change to index.
5. A damaged index. After a crash mid-write or a full disk, mds can keep retrying the same files. A restart often clears it; the rebuild step below is the definitive fix.
6. A bug in a specific version. Spotlight has had loops and leaks in several macOS releases, reported from Leopard onward. Updating is on the list for a reason.

What normal looks like, and what is not

How the three Spotlight processes typically share the load during an index, and what a stuck one looks like:

Two things make a healthy index look slower than it is. Spotlight throttles itself while you are actively using the Mac and speeds up when you walk away, so a job that seems stalled while you watch it finishes once you leave. And on a laptop, indexing is paused or slowed on battery and stops when the lid closes. A big re-index that only gets an hour of idle, plugged-in time each day can drag across a week of calendar time and still be healthy.

To see progress: open Spotlight with ⌘Space and type anything. While indexing, it shows an "Indexing" bar with the volume name. When that bar is gone and mds is still high the next day, it is time to act.

Is it safe to quit mds?

Quitting it in Activity Monitor does nothing harmful: launchd restarts it within a second and it resumes. It also does nothing useful, because the queue of files is still there. You cannot stop Spotlight by quitting its manager.

Turning Spotlight off altogether, with the Terminal commands that circulate in old threads, is the thing to avoid. It breaks search in Finder, Mail and the Open dialog, and some apps that rely on metadata start misbehaving in ways that do not obviously point back to Spotlight. The 2007 thread's original poster described the frustration everyone feels: "I'm having problems with the 'mds' process running out of control." The fix then and now is the same: give it less to index, not switch it off. The decision tree covers the general rule:

For mds specifically, the safe lever is the Search Privacy list in System Settings. Everything else is waiting or restarting.

What to do, in order

1. Plug in and leave it alone for an hour

   Most mds complaints are a normal re-index after an update that the person kept interrupting. Power on, lid open, walk away. Check Activity Monitor afterwards; usually it is back under 5 percent.

2. See which volume it is indexing

   Open Spotlight with ⌘Space, type a word, and look for the Indexing bar and the drive name. That tells you whether it is your internal disk or something you connected.

3. Exclude the drive or folder that keeps triggering it

   System Settings, Spotlight (Siri & Spotlight on older versions), scroll to Search Privacy, click the plus button, add the Time Machine disk, camera card, virtual machine folder, code checkout or whatever keeps feeding it. Indexing of that location stops immediately. You lose search inside it, nothing else.

4. Pause the app that writes constantly

   Sync clients, large mail downloads and running VMs generate endless changes. Pause the app for an hour so the index can catch up, then resume.

5. Restart

   Clears a daemon stuck on one bad file. If CPU drops and stays down, done.

6. Update macOS

   Spotlight loops and leaks have been fixed in point releases repeatedly. System Settings, General, Software Update.

7. Rebuild the index, last resort

   In Search Privacy, add your startup disk (Macintosh HD), wait a minute, then select it and click the minus button. Spotlight deletes and rebuilds the index from scratch. This takes an hour or more and search is incomplete until it finishes, so do it plugged in, overnight, and only after the steps above.

When to worry

mds is one of the processes Oakley lists as a "regular offender" that is still normal. The cases where it is telling you something:

• Busy every time a specific drive connects. That drive's index is damaged or the drive is failing. Exclude it, and if it is also slow in Finder, back it up now.
• Search returns nothing or stale results while mds is calm. The index is broken rather than busy. Rebuild it.
• Heat and battery. Hours of indexing on a laptop on a soft surface brings in kernel_task throttling and an empty battery. Do the index on a desk, on power.
• It never ends with nothing new to index. Rebuild. If even that fails, something is writing to disk constantly. Safe to Quit records each process's disk writes over hours, so the app that kept Spotlight busy all night shows up by name in the morning.

What is not a worry: all three Spotlight processes at the top of Activity Monitor for half an hour after an update. That is the index being built, and it is the reason search is instant the rest of the year.