Memory and Sizing

If I create an 8 GB RAMDisk, is 8 GB of my RAM gone?

For the default tmpfs backend: no. The size you choose is a ceiling, not a reservation. An 8 GB tmpfs disk holding 300 MB of files uses roughly 300 MB of RAM. Memory is consumed as files are written and released as the kernel reclaims it — so it's fine to size a scratch disk generously "just in case" as long as you don't actually fill it.

APFS and HFS+ volumes behave like fixed-size disks (their reported capacity is the full size regardless of content). If minimal memory footprint is the goal, stay on tmpfs.

The corollary of "ceiling, not reservation": a disk that fills up really does consume that much RAM. Filling a huge RAMDisk can push the system into memory pressure — the gauge in the app's bottom bar exists to warn you before that happens.

What are the size limits?

• Minimum: 16 MB.
• Maximum: 75 % of the machine's physical RAM — a guardrail, not a recommendation. macOS itself, your apps, and file caches all need RAM; see the gauge before creating anything near the limit.

When you create a disk from an existing folder, the app proposes the folder size plus ~10 % headroom and won't let you dial below the folder's content (you can't create a disk the content doesn't fit in). If the folder itself is larger than 75 % of RAM, creation is off the table on that machine.

How does the memory gauge decide what's "available"?

It approximates what Activity Monitor calls available memory: free memory, plus memory the system can reclaim instantly (purgeable and inactive file-cache pages), minus what's held by the memory compressor, minus a 512 MB safety margin so creating a disk doesn't immediately tip the machine into pressure. Your mounted RAMDisks' own usage is tracked on top of that. It refreshes every few seconds — and immediately after each mount or eject.

If the gauge shows less "available" than you expected, remember that macOS deliberately keeps RAM full of caches; that memory still counts as available here because the system will hand it back when your RAMDisk needs it.

Can I resize a volume? What happens to the content?

Yes, from the volume's settings:

• Growing a volume, or shrinking while the content still fits, preserves content. On a mounted volume the app remounts to apply the new size — with Save to Disk semantics, so content rides through via the usual save/restore.
• Shrinking below the space in use cannot preserve content: the app warns you explicitly, and confirming erases the disk and remounts it empty. This is the one resize path that destroys data — the warning is not boilerplate.

Should I keep disks mounted all the time?

Mounted disks hold their content in RAM even when idle, so the honest answer is: mount what you use, eject what you don't. With Save to Disk enabled, eject is cheap and safe (content lands in the folder; next mount restores it), and unmounting returns the memory to the system immediately.

Do RAMDisks survive sleep?

Yes. Sleep keeps RAM powered; your mounted disks and their contents are intact on wake. The dangerous transitions are the ones that cut power to RAM: shutdown, restart, power loss, and (on Macs configured to hibernate rather than sleep) a battery running completely flat — treat those as the crash scenario in Data safety unless a clean eject ran first.