Linux's file system check utility fsck is little recognised and largely unloved, no doubt because it seems to know whenever you're in a hurry to boot your machine. Then - and why is it only then? - it starts doing what it was designed for: checking the consistency of the filing systems on your hard drives and making any necessary repairs.

Since Ubuntu 9.04 users have at least had the option of hitting the Esc key to cancel the process, (users of earlier versions might like to consider Bonager), but what's actually going on and why does the checking seem so intermittent?

All file systems accumulate errors over time, often the result of application crashes, improper shutdown or bugs. Checking and recovery tools should be run periodically and fsck simply automates the process by setting a counter and checking off how many times each filing system has been booted. (It has to be done at boot as fsck can't be run over a mounted partition.) And that apparent randomness actually isn't. fsck typically sets the default boot count at 30 meaning that with daily boots your file systems will be checked once a month.

In Linux of course you can control everything. If you want all your file systems checked next time you boot simply enter the command;

sudo touch /forcefsck

That simply adds an empty file to the root directory ("/") called forcefsck, the presence of which instructs the kernel to check everything.

You can also change the checking interval or even turn it off completely, but before you do anything else type

sudo fdisk -l

to remind yourself of what's mounted where.

The key command in controlling when fsck runs is tune2fs. You can use either count-dependent or time-dependent checking;

sudo tune2fs -c 15 /dev/sda1

will check the filesystem on /dev/sda1 every 15 mounts while

sudo tune2fs -i 60d /dev/sda2

will check the filesystem on /dev/sda2 every 60 days. (This interval could also be written as -i 8w - every 8 weeks or -i 2m - every 2 months.)

You can even turn off checking completely by setting either the count or interval to 0 or -1

sudo tune2fs -c 0 /dev/sda3

but this is not recommended!


The -C option is used to set the number of times the file system has been mounted, meaning you can use it to stagger when they're due to be checked. Assuming you've just re-booted after setting sudo touch /forcefsck ...

sudo tune2fs -C 1 /dev/sda1 sudo tune2fs -C 2 /dev/sda2 sudo tune2fs -C 3 /dev/sda3

... will change the counts so that the partitions will be checked on consecutive boots, not all at once.


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