Why is my /tmp/ directory suddenly only 1MB big?

Today I got a really weird error on my Debian "Squeeze" Linux box -- a processes tried to write a temp file, and it complained that there was No space left on device.

The weird thing is, just yesterday my root parition was full, and I had made about 7GB free space in it.

I checked, there was still plenty of room today. But behold:

$ df -h /tmp/
Filesystem            Size  Used Avail Use% Mounted on
overflow              1.0M  632K  392K  62% /tmp

So, suddenly my /tmp/ directory was a ram disc with just 1MB of space. And it didn't show up in /etc/fstab, so I had no idea what cause it.

After googling a bit around, I found the likely reason: as a protection against low disc space, some daemon automatically "shadows" the current /tmp/ dir with a ram disc if the the root partition runs out of disc space. Sadly there's no automatic reversion of that process once enough disc space is free again.

To remove the mount, you can say (as root)

umount -l /tmp/

And to permanently disable this feature, use

echo 'MINTMPKB=0' > /etc/default/mountoverflowtmp