A deep dive into Debian 13 /tmp: What's new, and what to do if you don't like it

Summary

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 SolutionsNow 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 ...