A Deep Dive Into Debian 13’s /tmp: What’s New, And What To Do If You Don’t Like It - LowEndBox

Debian 13 “Trixie” introduces an important change to /tmp.  Traditionally, it’s been just another filesystem, albeit with some special permissions that allows everyone on the system to use it without being able to remove each other’s files.

In Trixie, it’s been moved off the disk into memory – specifically a type of memory called tmpfs.  To quote the tmpfs man page:

The  tmpfs  facility  allows  the  creation of filesystems whose contents reside in virtual memory. Since the files on such filesystems typically reside in RAM, file access is extremely fast.

They’re also extremely temporary…which is what you really want.  There’s an old story about a user who was assigned to work on the Transportation Management Project.  He logged into the server where he was supposed to store his work, saw the /tmp directory, found he could upload files there, and happily spent a couple months putting all his work there.  Alas, when the server was rebooted…

Now that is undoubtedly an urban legend, but it illustrates the true nature of /tmp.  It’s fine if you need a disposable log fine, a PHP session file, space for sorting something, etc.  But you shouldn’t be storing anything there.

This isn’t a new thing in the Linux world.  RedHat and its ilk have used tmps for /tmp for some time.

A more serious problem than people losing files is people who use too much /tmp.  The system needs /tmp to do basic functions, so if it hits 100%, things will break.  It’s really easy to think “I’m going to download and untar this big zip file into /tmp, and then I’ll remove it after I pull out the one file I need”…and forget to remove it.  Now you’re hogging /tmp and over time, /tmp can be filled up with junk.

Debian 13’s tmpfs Comes With…Challenges.  And Solutions

Now instead of filling up disk, you’re filling up memory.  If you download a 300MB .zip file, expand it to 1GB, and forget it, now you’re chewing up 1GB of RAM.  Ouch.

There are two mitigating factors.  First, by default, Debian will only allocate a maximum of 50% of RAM to the tmpfs for /tmp.  You can change this.  To do so, type

systemctl edit tmp.mount

You’ll be popped into your editor (controlled by the EDITOR environment variable) with a form to update the settings.  At the very bottom you’ll see a template, which you can copy and edit:

# [Mount]
# What=tmpfs
# Where=/tmp
# Type=tmpfs
# Options=mode=1777,strictatime,nosuid,nodev,size=50%%,nr_inodes=1m

Go back up to the part before the line “Edits below this comment will be discarded” and paste in something like this:

[Mount]
What=tmpfs
Where=/tmp
Type=tmpfs 
Options=mode=1777,strictatime,nosuid,nodev,size=25%%,nr_inodes=1m

to change it to 25% or if you want a number:

[Mount]
What=tmpfs
Where=/tmp
Type=tmpfs 
Options=mode=1777,strictatime,nosuid,nodev,size=1G,nr_inodes=1m

to change it to 1GB.

For example, I have a Debian 13 VPS with 4GB of RAM.  After a fresh install, I see it’s using 2GB max for tmpfs:

# findmnt --target /tmp
TARGET SOURCE FSTYPE OPTIONS
/tmp   tmpfs  tmpfs  rw,nosuid,nodev,size=2007704k,nr_inodes=1048576,inode64

Note that this is a maximum.  If there’s nothing in /tmp, /tmp does not use any memory.

After doing the systemctl edit, like this:

I get the message:

Before this change, /tmp was at 2GB (half of the 4GB RAM):

Now, after reloading systemd and restarting tmp.mount, I see /tmp is limited to 1GB:

Cleanup

The second mitigating factor is that /tmp is now automatically cleaned up.  Quoting the release notes:

The new default behavior is for files in /tmp to be automatically deleted after 10 days from the time they were last used (as well as after a reboot). Files in /var/tmp are deleted after 30 days (but not deleted after a reboot).

You can modify these policies, exclude certain files (why?  they’re temporary!), or even apply it to other directories.  Consult the fine manual but I think for 99% of people, the defaults are just fine.  I might be tempted to make the cleanup a little more aggressive, like 3 days.

Thinking in a LowEnd Context

One concern is for very low-memory systems.  While 1GB has become the smallest VM for a lot of people, 512s are still sold.  Allowing /tmp to consume 256MB out of 512 (which is really only 470-480 after the kernel and vital system processes are loaded) is a lot more impactful than consuming 256MB on a 10GB or 20GB filesystem.

Fortunately, opting out of the new tmpfs world is easy if you don’t like it:

systemctl mask tmp.mount

and reboot.  I did that on the test box above:

Now I can put 17GB of junk there.  Fortunately, it will be cleaned up as described above.

So how are you planning to handle Debian 13’s new tmpfs-based /tmp?