How to use fsck to find and repair disk errors & bad sectors

Use fsck to find and repair disk errors and bad sectors

Fsck

The “File System Consistency Check” is the full form for “fsck”, It is a utility that examines the file system for errors(in Unix and Unix-like operating systems, such as Linux, macOS, and FreeBSD) and attempts to repair them if possible. It uses a combination of built-in tools to check the disk and generates a report of its findings.

On some systems, fsck runs automatically after an unclean shutdown or after a certain number of reboots.

When to Use fsck in Linux

Use fsck to check the file system,

• If the system fails to boot.
• If files on the system become corrupt (often you may see input/output error).
• If attached drive (including flash drives/SD cards) is not working as expected.

Will fsck delete files

fsck does not touch the files. It is basically a front-end program that does all kinds of filesystem checks (ie. it checks the integrity of the journaling system). Besides checking inodes and blocks fsck also checks for incorrect

fsck Options and Arguments

Option	Action
-a	Attempt to fix errors automatically. Use with caution.
-f	Force fsck to check a file system even if it thinks it’s clean.
-A	Check all disks listed in /etc/fstab.
-C	Show progress bar (ext2 and ext3 file systems only).
-M	Skip mounted file systems.
-N	Test run. Describes what would happen without executing the check itself.
-P	Use with the -A option to run multiple checks in parallel.
-R	If using the -A option, do not check the root filesystem.
-t	Check only a specific type of filesystem.
-T	Skip the title on startup.
-y	Interactive repair mode.

How to Run fsck to Repair Linux File System Errors

In order to run fsck, you will need to ensure that the partition you are going to check is not mounted. The easiest and safest way to unmount your disk is to use Rescue Mode. You need to enter in to rescue mode via either single user mode or by uploading and booting rescue ISO on Cloud Server.

What is Rescue Mode

The rescue mode provides the ability to boot a small Red Hat Enterprise Linux environment entirely from CD-ROM, or some other boot method, instead of the system’s hard drive. As the name implies, rescue mode is provided to rescue you from something. By booting the system from an installation boot CD-ROM

If you are using fsck, the easiest and safest way to unmount your disk is to use Rescue Mode. Also the fsck should be run only as a user with root permissions.

How to view mounted Disks and their Location.

‘df’ command helps to view a list of currently mounted disks. If you are in Rescue Mode, the disk you want to check should not be listed in the output.

df -h

root@test:~# df -h
Filesystem Size Used Avail Use% Mounted on
tmpfs 739M 1016K 738M 1% /media/ramdisk
/dev/sdh 160M 160M 0 100% /media/sdh
/dev/loop0 146M 146M 0 100% /media/compressed_root
unionfs 739M 1016K 738M 1% /
devtmpfs 10M 0 10M 0% /dev

Run fdisk to view disk locations:

fdisk -l

Then copy the location of the target disk to use with the fsck command.

How to unmount manually the partition

Use the command umount to unmount the disk location copied in the last step. For example take it as /dev/sda

umount /dev/sda

If the disk is declared in /etc/fstab, change the mount point to none there as well.

How to Check for Errors on a Disk

Run fsck on the target disk, using the desired options. For example, check all file systems (-A) on /dev/sda:

fsck -A /dev/sda

After running fsck, it will return an exit code. The description of the codes are as follows:

Code	Code Meaning
0	No errors
1	Filesystem errors corrected
2	System should be rebooted
4	Filesystem errors left uncorrected
8	Operational error
16	Usage or syntax error
32	Checking cancelled by user request
128	Shared-library error

Repair Linux Filesystem Errors

Sometimes more than one error can be found on a filesystem. In such cases run fsck to automatically attempt to correct the errors. This can be done with:

fsck -y /dev/sda

To run the same on all filesystems (without root):

fsck -AR -y

Performing a File System Check

To run fsck by entering the following command, replacing /dev/sda with the location of the disk you want to check and repair:

e2fsck -f /dev/sda

If no problems are detected, fsck will display the tests it performed

e2fsck -f /dev/sda
e2fsck 1.42.13 (13-Sept-2019)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/sda: 109771/1568000 files (0.5% non-contiguous), 675014/6422528 blocks

If fsck determines that there is a problem with the filesystem, it will prompt you to fix problems as they are found during each test:

root@ttyS0:~# e2fsck -f /dev/sda
e2fsck 1.42.13 (13-Sept-2019)
ext2fs_check_desc: Corrupt group descriptor: bad block for block bitmap
e2fsck: Group descriptors look bad… trying backup blocks…
e2fsck: Bad magic number in super-block while using the backup blockse2fsck: gok
Superblock has an invalid journal (inode 8).
Clear?

Once the filesystem check completes, any problems detected should be fixed.

At last, to exit from rescue mode. Try reboot your server, If fsck fixed the issues, the server should boot normally.

reboot

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