What triggers file system check?

Question

What are the conditions that trigger file system check at system startup in Ubuntu 11.10?

Answer 1

There's a few conditions which will trigger a fsck when a filesystem is mounted.

Firstly the maximum mount count, and the check interval. To see these, try:

sudo tune2fs -l /dev/sda1

Replace sda1 with the filesystem you want to check.

You're looking for:

Mount count:              9
Maximum mount count:      29
Last checked:             Tue Nov 15 17:15:10 2011
Check interval:           15552000 (6 months)
Next check after:         Sun May 13 18:15:10 2012

Here you can see my drive has been mounted 9 times since it was last checked, and it will force a check after 29 mounts just to make sure everything is still okay. It will also do one after 6 months even if the maximum mount count hasn't been reached. (these values are also tweakable with tune2fs).

Secondly, if the filesystem is not cleanly unmounted, the next mount attempt will trigger a fsck. This is also in the tune2fs output, look for:

Filesystem state:         clean