/tmp on tmpfs by default

Debian 13 (Trixie) mounts /tmp as an in-memory tmpfs by default: faster temporary files, cleared on reboot. What changes, what to watch for, and how to revert.

Starting with Debian 13 (Trixie), the /tmp directory is mounted as a tmpfs by default (an in-memory filesystem, mounted through systemd's tmp.mount with rw,nosuid). Temporary files no longer hit the disk and are cleared automatically on reboot.

What changes

	Debian 12 and earlier	Debian 13
Where `/tmp` lives	Disk (root filesystem)	Memory (tmpfs)
After a reboot	Files remain	Cleared automatically
Read/write speed	Limited by the disk	Close to memory speed
Space limit	Free disk space	About half of RAM by default (tmpfs default, used on demand)

Faster — build artifacts, extraction and caches do their temporary I/O in memory
Cleaner — a reboot wipes /tmp, so no stale temporary files pile up
Kinder to SSDs — frequent temporary writes stop wearing the disk

/var/tmp stays on disk — temporary data that must survive a reboot (some crash dumps, packaging build trees) belongs there.

What to watch for

Files in /tmp do not survive a reboot

Any workflow that assumes files "stay in /tmp until I delete them" breaks. Half-downloaded files and unfinished script output are gone after a reboot. If you park important files in /tmp out of habit, it's time to change that habit.

Large files — tmpfs content lives in memory (and can swap out). Writing multi-gigabyte files into /tmp on memory-constrained devices can create memory pressure
Programs that assume /tmp is persistent — the rare software that expects /tmp to survive reboots should switch to /var/tmp

Inspect and adjust

# Confirm the mount type of /tmp
findmnt /tmp
# Output looks like: tmpfs /tmp tmpfs rw,nosuid,...

# Capacity and usage
df -h /tmp

Revert to a disk-backed /tmp

If you want /tmp back on disk (small-memory machines, or services that need a huge /tmp):

# Mask the tmp.mount unit and /tmp falls back to the root filesystem (disk)
sudo systemctl mask tmp.mount

Takes effect after a reboot. To go back to the default tmpfs, unmask:

sudo systemctl unmask tmp.mount

Resize the tmpfs

Add an /etc/fstab entry for /tmp to override the default size (fstab entries take priority):

tmpfs /tmp tmpfs rw,nosuid,nodev,size=4G 0 0

Next steps

Debian 13 hub - back to the release overview
Disk & mount management - fstab and mounts in depth
Basic configuration - more system tuning