Alpine backups: Home directory

I set up a cheap NAS-alike using a Raspberry Pi. After a first attempt using the default OS resulted in an unusable SD card, I did some digging and decided to use Alpine Linux, mostly because it runs by default in Diskless Mode.

In Diskless Mode, the SD card is consulted at boot time, when the entire file tree is loaded into memory (using tmpfs), and then it is left alone. All operations that write to disk (logging, etc.) go to memory. The next time the device boots, it comes up the same way. This effectively turns the server into an appliance, and makes the reboot operation return to a known good state.

However, we do want to change some things from the default, and for this, Alpine provides Alpine Local Backup, which lets users save modifications to an overlay file on the SD card, so they “survive” the next reboot.

By default, the only changes it “notices” are inside /etc, but you can include more directories to “watch”.

This largely works as advertised, but one of the hitches I ran into was how to include a user’s home directory, without all the files in it.

Here’s how I did it.

The story so far

Running everything as root is a bad idea. Instead, I’ve created a shell user to do my day-to-day operations. (It’s called huevo, ‘cause eggs have shells 😬).

$ adduser huevo

Now we explicitly include the home directory, so that it survives reboot (with the appropriate permissions):

$ lbu include /home/huevo

Cleaning up

So far, so good. But now the home directory picks up any changes we make, which will include things like .ash_history, .viminfo, and other cruft. To prevent inadvertent bloat of the overlay file, we don’t want to pick up those things by accident. Instead, we want include only those files that we’ve explicitly chosen to stick around.

This is where the documents aren’t directly helpful: Running lbu exclude /home/huevo/* actually excludes the home directory as well.

The solution

It turns out that lbu stores its list of added paths in /etc/apk/protected_paths.d/lbu.list. After playing around with the file (and running lbu ls a bunch), it turns out that the order matters! Switching around the exclude and include statements makes the problem go away:

$ cat /etc/apk/protected_paths.d/lbu.list
-home/huevo/*
+home/huevo

This pattern also works for other paths, like persisting the ~/.cache folder without any of the cached files.