“Optimise Mac Storage” is not a cleanup button

What the setting actually changes

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

With it on, macOS may evict the local copy of a file. What remains is a zero-byte placeholder with a cloud icon in the Finder. Opening it downloads it again. Nothing has been deleted from iCloud, and nothing is lost, provided you have a network connection at the moment you need the file.

The rules macOS uses to choose are not documented in detail, but the observable behaviour is consistent:

• Eviction happens under disk pressure, not on a schedule. Plenty of free space means nothing is ever evicted, which is why turning the setting on frequently frees nothing at all.
• Least recently opened files go first, and larger ones are preferred among equals.
• Files you have opened in the last few days are left alone.
• Photos has a separate setting with the same name and its own rules — that one replaces full-resolution originals with optimised versions.

Why it does not help when you need it

This is the common complaint, and it follows directly from the first rule. People turn the setting on because the disk is full, expect it to reclaim space, and see the number move by very little. Three reasons:

1. Eviction only applies to files that are fully uploaded to iCloud. Anything still uploading, or anything that failed to upload, is pinned locally and cannot be evicted.
2. It only applies to iCloud Drive content. If your disk is full of Xcode build output, Docker images or local AI models, none of it is eligible, and those are usually where the space went. Checking why the System Data figure never adds up is a better first move.
3. The space it does free is often reported as purgeable rather than free, so the Finder’s number barely changes even when eviction has happened.

Telling what is actually on the disk

The Finder shows a cloud icon next to online-only files, which is easy to miss on a long list. From the command line, the distinction is visible as a size difference between the logical file and its allocated blocks:

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

The .icloud extension is the old placeholder format and still appears in some contexts. On current macOS the more reliable check is du against ls -l: a file whose apparent size is 2 GB and whose disk usage is zero is evicted. Telling online-only files from local ones, across every sync client covers doing this across Dropbox, OneDrive and Google Drive too, all of which use different mechanisms for the same idea.

Before travelling, open the folders you will need while offline. An evicted file on a plane is a file you do not have, and there is no warning until you double-click it.

Forcing files up or down

macOS has explicit commands for both directions, which are more reliable than hoping the policy does what you want:

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

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

Right-clicking a file in the Finder offers “Remove Download” and “Download Now” for the same two operations. Use those when you want a specific 40 GB folder gone from the disk rather than waiting for macOS to decide.

When to leave it off

Turn it off if any of these apply, because the failure mode is worse than the space saved:

• You work offline regularly, or on unreliable connections.
• Your iCloud Drive contains working files for an application that opens many at once — a photo catalogue, a code repository, a design library. Bulk re-download at the wrong moment is painful.
• You rely on a local backup. Time Machine backs up the placeholder, not the file, so an evicted file is not in your backup either. That is the single most important consequence of this setting and it surprises almost everyone. What to back up before any cleanup takes it further.

Questions

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