Turn on Optimise Mac Storage for iCloud Drive and something odd happens: files you can see in Finder, that look completely normal, don't actually take up space on your disk. macOS calls these placeholder or "dataless" files — the file's name, icon, and metadata are all present locally, but the actual contents live only in iCloud until you open the file, at which point macOS quietly downloads it in the background. It's a genuinely useful feature on a laptop with a small drive and a big iCloud library, but it comes with a trade you should understand before you turn it on: you're exchanging disk space for a dependency on your network connection, and that's a bad trade in specific, predictable situations.

What a dataless file actually is

When Optimise Mac Storage is on, macOS looks at which iCloud Drive files you haven't opened recently and evicts their contents from local disk, replacing them with a lightweight placeholder. In Finder, evicted files show a small cloud-with-down-arrow icon overlaid on the file. Double-click one and Finder starts downloading it immediately; how long that takes depends entirely on the file's size and your connection speed. Files you've opened recently, or explicitly marked to keep local, stay fully downloaded.

This is different from a shortcut or an alias. The file still behaves like a real file to most apps — it has the correct file size reported by the Finder, the correct modification date, the correct name — it just doesn't have its bytes sitting on your SSD until something asks for them.

Where to control it

The toggle lives in Apple menu → System Settings → [your name] → iCloud → iCloud Drive. From there you can turn Optimise Mac Storage on or off for the whole iCloud Drive. The Storage Settings recommendations panel (About This Mac → More Info → Storage Settings → Recommendations) surfaces the same setting as a suggestion when your disk is getting full, phrased as "Store in iCloud."

You can also override the setting per-file or per-folder: right-click any file or folder in iCloud Drive and choose Download Now to force it local permanently, or in some macOS versions, right-click and remove the local copy to push it back to placeholder status manually.

When it's the wrong trade

Optimise Mac Storage is a reasonable default for most people most of the time, but there are situations where it actively works against you:

  • A slow or metered connection. Every file you open that isn't already local triggers a download. On a slow hotel Wi-Fi or a mobile hotspot with a data cap, this turns "open a document" into "wait, and possibly pay for the privilege."
  • No connection at all. On a flight, in a basement server room, or anywhere genuinely offline, a placeholder file simply cannot be opened. If you know you'll need a folder of documents offline, download it explicitly beforehand rather than assuming it'll be there.
  • Batch operations on many files. Running a script, an export, or a search across a folder full of placeholders can be slow and unpredictable, because each file access potentially triggers a network round trip.

If any of these describe how you're about to use your Mac, it's worth checking which folders you actually need local before you go, not after.

Why a storage tool can under-report an iCloud Drive folder

This is the part that trips people up when they compare numbers between tools. A folder full of placeholder files has almost no local bytes — the actual file contents simply aren't on disk. A tool that measures "how much space does this folder use on this disk" (which is what du -sh <path> and most storage visualisers report) will correctly show a small number for that folder, because that's genuinely how much of your disk it's using right now.

The confusion comes from expecting that number to match the "real" size of the folder — the sum of every file's full, logical size, as if everything were downloaded. Those are two different, both-correct answers to two different questions: "how much of my disk does this use" versus "how big is this content in total." A tool that only ever answers the first question isn't wrong, but if you don't know that's the question it's answering, its number for an Optimise-enabled iCloud Drive folder can look surprisingly small compared to what Finder's Get Info shows for the same folder's logical size, or compared to what the iCloud website says you're storing.

You can see this directly:

du -sh ~/Library/Mobile\ Documents/com~apple~CloudDocs

If most of that folder's files are placeholders, this number reflects mostly local bytes plus overhead, not the full amount of data you have stored in iCloud.

The other direction: forcing everything local can fill your disk

The inverse trap is just as real. If you turn Optimise Mac Storage off, or select a large folder and choose Download Now, macOS starts pulling every placeholder's full contents onto your disk. If your iCloud library is larger than your remaining free space — which is exactly the situation that made Optimise Storage attractive in the first place — you can run your disk to full during the download, sometimes before you notice what's happening. Before forcing a large folder local, check Storage Settings for how much free space you have and compare it to the total size iCloud reports for that folder online or in the Files app, not just what's currently on disk.

Conclusion

Placeholder files are a sensible piece of engineering for the common case: most files, most of the time, sit unopened, so there's no reason to keep every byte local. The cost only shows up at the edges — slow networks, no networks, and the moment you decide to reverse the setting on a library bigger than your free space. Knowing which regime you're in before you flip the toggle avoids all three problems.

We built VolumeLens because we needed a storage map that tells you what it's actually measuring, rather than presenting one number as if it were the only truth. It reports on-disk usage honestly, including flagging where files may not reflect their full logical size, so you're not left guessing why an iCloud Drive folder looks smaller than you expected. You can grab it and see for yourself on the download page.