"no space left on device" during docker build

TL;DR: Free docker disk usage with docker system prune -a --volumes (careful: deletes unused images, containers, networks, and volumes), or move /var/lib/docker to a bigger disk. Check usage first with docker system df to see what is eating space. Builds need room for every layer plus temp files, and a full docker dir fails mid-build.

The error

failed to solve: write /var/lib/docker/tmp/buildkit-mount123456: no space left on device

Fix it

1. See what docker is holding:

docker system df Expected: shows Images/Containers/Volumes sizes and reclaimable amounts.

1. Prune what you do not need (confirm each prompt, or add -f):

docker system prune -a --volumes Expected: reclaims GBs; shows totals per type.

1. Retry the build.
2. If the disk itself is small, move docker's data dir: stop docker, mv /var/lib/docker /newdisk/docker, symlink or set "data-root": "/newdisk/docker" in daemon.json, start docker.

When this applies

• Builds failing with no space while df -h / shows /var/lib/docker full
• CI runners accumulating images

When this does NOT apply

• "failed to register layer: no space left on device" on PULL (same disk, but prune images first since the pull cannot complete)
• Host root full from non-docker causes (logs, etc.; check du -sh /*)

Versions

All Docker versions.

Why it happens

Every build layer, build cache entry, and temp file lives under /var/lib/docker. BuildKit keeps aggressive caches, and docker system df often shows tens of GB reclaimable on busy machines.

Edge cases

• docker builder prune clears just the build cache without touching images; safer first step.
• Overlay2 with many small layers wastes space; multi-stage builds that copy between stages keep final images small.
• Never delete /var/lib/docker files by hand while the daemon runs; use the prune commands or stop the daemon first.