# What "Optimise Mac Storage" really does to your files

> The iCloud setting that quietly evicts your files to the cloud, the rules macOS uses to choose them, why it often frees nothing when you need it to, and how to tell what is actually on your disk.

Canonical: https://bytesweep.app/blog/icloud-optimise-storage.html
Last updated: 2026-09-29

## “Optimise Mac Storage” is not a cleanup button

“Optimise Mac Storage” sounds like a task you run. It’s a standing policy instead: it tells macOS that when the disk gets tight, it may remove the local copy of any file that’s already safely in iCloud Drive, and leave a placeholder that downloads again when you open it. The setting gets blamed both for losing files and for freeing nothing. Neither is quite what happens.

- By [Shajeen Ahamed](https://bytesweep.app/about.html#author)
- Published 28 September 2026
- Last updated 29 September 2026
- 3 min read
- Part of [Disk space analyzer for Mac](https://bytesweep.app/mac-disk-space-analyzer.html)
- [How we write](https://bytesweep.app/about.html#how-we-write)

### What it changes

With the setting off, everything in iCloud Drive (and Desktop and Documents, if you sync those) exists twice: once in iCloud, once on your disk.

With it on, macOS may evict the local copy. The file stays in the Finder with a cloud icon, keeps its name and size, and has no data on the disk. Opening it downloads it again. Nothing is deleted from iCloud and nothing is lost, as long as you have a connection at the moment you need the file.

Apple doesn’t document how macOS chooses what to evict. What you can observe is that it happens under disk pressure rather than on a schedule, and that files you haven’t opened for a while go before ones you’re using.

Photos has its own setting with the same name, and it’s a different mechanism: Photos replaces full-resolution originals with smaller versions and keeps the originals in iCloud. [Inside a Photos library](https://bytesweep.app/blog/photos-library-size.html) covers what that leaves on disk.

### Why switching it on often frees nothing

People usually turn the setting on because the disk is already full, and then watch the number barely move. Each file has to get through a few gates before it’s evicted, and most of what fills a full disk fails one of them.

*A file has to pass every gate before it leaves the disk, and even then the Finder may call the space purgeable.*

- No pressure, no eviction. With plenty of free space, nothing is ever evicted, so turning the setting on changes nothing that day.
- Only iCloud Drive content is eligible. Xcode build output, Docker images and local AI models aren’t in iCloud, and on a full developer Mac that’s usually where the space went. [Why the System Data figure never adds up](https://bytesweep.app/blog/system-data-explained-numbers.html) is a better first stop.
- Only fully uploaded files. Anything still uploading, or stuck after a failed upload, stays pinned to the disk.
- Freed space often appears as purgeable before it appears as free, so the Finder’s number moves less than the eviction did.

### Evicting a folder yourself

If you know which 40 GB folder you want off the disk, don’t wait for the policy to decide. `brctl` does it directly, in both directions:

```
## Evict the local copy, keeping it in iCloud
brctl evict ~/Documents/big-project

## Pull everything back down
brctl download ~/Documents/big-project
```

In the Finder, right-click and choose “Remove Download” or “Download Now” for the same two operations.

### Seeing what’s actually on the disk

The Finder’s cloud icons are easy to miss in a long list. From the Terminal:

```
find ~/Library/Mobile\ Documents -type f -name '*.icloud' | head
du -sh ~/Library/Mobile\ Documents/com~apple~CloudDocs
```

Files ending in `.icloud` are the older placeholder format, which you may still find on Macs that have been upgraded over the years. On current macOS an evicted file keeps its real name, so compare `du` with `ls -l` instead: a file that claims 2 GB and uses no disk is evicted. [Telling online-only files from local ones](https://bytesweep.app/blog/cloud-files-online-only.html) does this across Dropbox, OneDrive and Google Drive as well, and each of them has its own settings for the same idea.

Before travelling, open or download the folders you’ll need offline. An evicted file on a plane is a file you don’t have, and there’s no warning until you double-click it.

### When I’d leave it off

The failure modes can be worse than the space saved, so I’d keep it off if:

- You work offline often, or on unreliable connections.
- iCloud Drive holds working files for an app that opens many at once, like a photo catalogue, a code repository or a design library. A bulk re-download at the wrong moment hurts.
- You count on a local backup. Time Machine backs up the placeholder, not the file, so an evicted file isn’t in your backup and your only copy is the one in iCloud. That’s the consequence of this setting that matters most, and it surprises almost everyone. What to back up before any cleanup goes further.

Turning it off later brings everything back down, in the background, over time. On a large iCloud Drive that’s hours of downloading and a lot of disk, so check you have the room before you flip it.

### Let Bytesweep clear this for you

Skip the commands. Bytesweep finds this for you, rates every item safe, review first or keep, and clears what you choose. Most of what it removes can be put back for seven days. See what it finds on your own disk, free for 3 days, then pay once with no subscription.

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### Keep reading

- [**Telling online-only files from local ones, on any sync client** iCloud, Dropbox, OneDrive and Google Drive all show a placeholder that looks like a file. One command distinguishes them across every client at once, and the distinction decides what is in your backup.](https://bytesweep.app/blog/cloud-files-online-only.html)
- [**Google Drive and OneDrive on a Mac: where the local copies hide** Both use macOS file providers now, and both keep caches outside the folder you can see. The real paths, how to force files online-only, and why the sync folder's size never matches the account's.](https://bytesweep.app/blog/google-drive-onedrive-mac.html)
- [**Dropbox online-only files and the space they still use** Dropbox can keep files in the cloud and show them locally, but the placeholders, the cache and the old-version store all cost real disk. Where each lives, how to make files online-only in bulk, and what the hidden cache holds.](https://bytesweep.app/blog/dropbox-smart-sync-disk.html)