A Short Rant on 15-122

Okay, this post isn’t anything truly substantive. It’s just a minor annoyance with the way the 15-122 course1 at CMU used to set things up, as well as a quick fix for the same.


Many courses at CMU make use of the Andrew Linux timeshare machines to provide a standardized computing environment for all students. Some courses also like to provide a setup script for students to run at the beginning of the semester—this is particularly true of introductory-level courses, where many students are probably unfamiliar with Unix systems. I personally wish that the setup process for these courses were a little cleaner than just running a foreign shell script; it’s usually not too bad, but having a setup tool similar to what checkinstall does for make install would be nice.

However, I want to call out 15-122 for having a particularly idiotic setup process.2 For some background, the Andrew Linux machines do a nightly backup of all user data. You can then choose to mount your backup directory on your file system, typically at ~/OldFiles. The stupid thing that the 15-122 script did was mount the backup at ~/private/15122/OldFiles. I have no clue what possessed them to do this, but besides being a very illogical place, it causes a very peculiar issue: it eats up one’s disk quota. Each user on the Andrew Linux timeshare is given a two-gigabyte quota. You can check your quota usage via:

$ fs listquota

Now ~/OldFiles is privileged in the sense that it doesn’t count against one’s quota usage, but ~/private/15122/OldFiles does. This means that if you followed the 15-122 setup instructions and now use the Linux timeshare for anything other than basic coursework,3 you can quickly run out of space. Fortunately, this is easily fixed by unmounting the volume:

$ fs rmmount ~/private/15122/OldFiles

But it’s still really annoying. If you ever need to access the backup, you can just mount it at its usual place using fs mkmount.


  1. Basically your standard introductory computer science course, for those not at CMU.

  2. In all fairness, 15-122 has since amended its setup script since I took the course, and this rant no longer applies.

  3. For instance, one can use the ~/www directory as a personal website or a quick-and-dirty file sharing site.