Monday, 29 September 2014

Definitive Guide to Recovering from a Full Disk

Cheap, stingy guy that I am, I allocate really small system partitions to my Ubuntu servers. This means that periodically my disk fills up. It fills up because every kernel upgrade takes a fair amount of space, and old kernels aren’t cleaned out automatically. Unfortunately, the disk usually fills up when trying to do an upgrade, so apt-get fails, and terminates with a partially installed package. You’ll know that has happened when you get a message like this when you run an apt command:
E: Unmet dependencies. Try using -f.
Once that happens, you can’t use any apt command.
There’s lots of advice out there about what to do, but the pages I’ve found always seem to leave something out, or assume knowledge of apt or dpkg that I don’t have.
So based on the last time this happened, here’s how I plan to recover the next time I run out of space. Warning: lots of Terminal commands coming up. I do everything in the Terminal for a few reasons:
  • The happens to me most often with servers, as I’m trying to save space, especially for virtual machines. My servers don’t have a GUI
  • Terminal works for both desktop and server machines
  • It’s easier to document commands for the Terminal
First, I have to make sure the problem really is that I’m out of space. (Looking for 0 in the “Avail” column, on the line that has “/” under the “Mounted on” column):
$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       3.7G  3.7G     0 100% /
Then I find out what version of the kernel I’m running:
$ uname -a
Linux ixmucane 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Next, I find out what kernels are installed:
$ dpkg --list | grep linux-image
If I have at least two versions older than the one I’m currently running, I can remove the oldest one (replacing the “n.nn.n-nn” with the version number I want to remove):
$  sudo dpkg --purge linux-headers-n.nn.n-nn-generic 
$  sudo dpkg --purge linux-headers-n.nn.n-nn
$  sudo dpkg --purge linux-image-extra-n.nn.n-nn-generic 
$  sudo dpkg --purge linux-image-n.nn.n-nn-generic 
This should free up lots of space, but I check again with df -h. Then run:
$ sudo apt-get -f install
If the amount of space it needs is less than what’s available according to df -h, then I go ahead and finish the install. To be safe, I also do:
$ sudo apt-get update
If there’s not enough space, and I have more old versions of the kernel installed, I just repeat the above dpkg until I have enough space to finish the install.
The above is the happy path. If I didn’t have two versions older than the current running kernel, I would try to remove the partially installed packages. Looking again at the output of:
$ dpkg --list | grep linux-image
if the newest version there is newer than the kernel currently running, then I would try the above dpkg commands to remove the partially installed packages. Some of them won’t work, of course, since the package isn’t installed, But once all the installed packages are removed, presumably there would more space and I could try:
$ sudo apt-get -f install
The reason I want to have two versions older than the current version is in case for some reason the current kernel doesn’t work, I can go back to the previous version. This is a cautious approach. If I’m really stuck, I would remove all versions except the current version. I’d probably make sure that I could boot the current kernel first. I haven’t had to do this and I hope I never do, but…