Docker Desktop on a Mac: where your memory and disk went, and how to get them back

Docker on a Mac is not quite Docker. Containers need a Linux kernel, and macOS does not have one, so Docker Desktop runs a small Linux virtual machine and runs your containers inside it. Almost every "Docker is eating my Mac" problem follows from that.

• Memory: the virtual machine has a memory limit, and Activity Monitor shows the whole machine as one large process, no matter how little your containers are doing.
• Disk: everything Docker stores, including images, containers, volumes and build cache, lives in one disk image file. It grows as you work and does not shrink by itself.

Memory: the virtual machine's limit

Open Docker Desktop's Settings › Resources. The memory limit there is how much the virtual machine may use, and by default it is half of your Mac's memory. On a 16 GB Mac that is 8 GB. On an 8 GB Mac it is 4 GB, before your editor, browser and dev server have started.

Three changes help, in this order:

1. Lower the limit to what you actually use. Run your normal stack, then run docker stats to see what each container really uses. Set the limit a little above the total. A Postgres and a Redis for local development rarely need more than 2 GB between them.
2. Turn on Resource Saver (in Settings › Resources, in current versions). It pauses the virtual machine after a period with no running containers, so an idle Docker uses next to nothing.
3. Quit Docker Desktop when you are done. Stopping containers frees memory inside the virtual machine. Quitting gives it back to macOS.

Disk: one file that only grows

Everything lives in Docker.raw, under ~/Library/Containers/com.docker.docker. Do not judge it by its size in Finder. It is a sparse file, so Finder may show the maximum it is allowed to reach rather than what is really written. Ask Docker:

docker system df

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          41        6         18.4GB    14.9GB (80%)
Containers      9         3         212MB     180MB (84%)
Local Volumes   17        4         6.1GB     4.8GB (78%)
Build Cache     233       0         9.7GB     9.7GB

docker system df -v breaks each line down image by image and volume by volume. The RECLAIMABLE column is the one to read. It is space held by things no running container uses.

What each prune deletes

Command	Removes	Safe?
docker builder prune	Build cache	Yes. Your next build is slower
docker container prune	Stopped containers	Yes, unless you meant to restart one
docker image prune	Dangling images (untagged leftovers of rebuilds)	Yes
docker image prune -a	Every image not used by a container	Yes, but you will download them again
docker system prune	All of the first three, plus unused networks	Yes
docker volume prune	Unused anonymous volumes. Add -a for named ones too	Careful. Volumes hold your databases

On most development machines, the build cache and old images are where the space is, and docker builder prune plus docker image prune -a gets most of it back. Volumes are where local databases live. Before pruning them, run docker volume ls and read the names.

Current versions of Docker Desktop return freed space to macOS by themselves, usually within a few minutes. If the space has not come back after a while, restart Docker Desktop.

Cap it

Under Settings › Resources, Docker Desktop also has a limit for the virtual disk. Setting it to something your Mac can afford turns "the disk filled up overnight" into a clear error from Docker, which is much easier to deal with.

When not to run Docker at all

If Docker on your Mac is there only to run a database, you may not need it. brew install postgresql@16 or brew install redis runs natively, with no virtual machine underneath. It starts in a second and uses a fraction of the memory. brew services start postgresql@16 runs it in the background, and brew services stop stops it.

Keep Docker for what really needs it: a multi-service stack, a production-like Linux environment, or a project where everyone has to run the same versions.

Where Stackyard fits

Docker's own commands are the right tools for the disk side, and Stackyard does not replace them. It does not delete images or volumes. Docker's data stays under Docker's control. What Stackyard does is the day-to-day management that keeps memory under control, without you going back to the terminal or opening Docker Desktop's window.

• The Docker section groups containers by Compose project, with CPU, memory, ports, image, health and logs for each. Start, stop or restart one container, or press Start All and Stop All for a whole project. If Docker is not running, Start Docker launches it.
• The Overview shows Docker's virtual machine beside everything else using memory. When memory is under pressure, it offers to quit the virtual machine, in one click.
• Localhost names the container behind each port, so localhost:5432 reads as your project's database rather than just a number.
• Homebrew services, the native alternative above, each get an on/off switch in the same section. Running Postgres without Docker is still one click.
• History charts memory for 24 hours, by app. A day's chart shows what Docker actually costs you, and what limit to set.

Every local server, with a health dot covers the Localhost and Docker screens in more detail. On a smaller machine, read web development on an 8 GB Mac next.

Stackyard is free for three days, then from $21 once for one Mac, with no subscription. At 3.9 MB, it is not adding to the problem. Download it from its page.