104.2. Maintain the integrity of filesystems

Weight: 2

Description: Candidates should be able to maintain a standard filesystem, as well as the extra data associated with a journaling filesystem.

Key Knowledge Areas:

  • Verify the integrity of filesystems

  • Monitor free space and inodes

  • Repair simple filesystem problems

Terms and Utilities:

  • du

  • df

  • fsck

  • e2fsck

  • mke2fs

  • debugfs

  • dumpe2fs

  • tune2fs

  • XFS tools (such as xfs_metadump and xfs_info)

In cases when your system crashes or loses power, your filesystems may be left in an inconsistent state, with some changes completed and some not.

Operating with a damaged filesystem is not a good idea as you are likely to further compound any existing errors.We’ll take a look at some tools to help us manage such problems.

You should always back up your filesystem before attempting any repairs!

fsck

The main tool for checking and repairing filesystems is fsck, which, like mkfs, is really a front end to filesystem-checking routines for the various filesystem types.

Some of these are just links to e2fsck command and they are the same

The fsck command in Linux allows us to manually check for file system inconsistencies, Fsck command needs to be run with superuser privileges or root(ubuntu 16.04 here):

In order to use fsck the partition should be unmounted, otherwise it might cause damages!

Lets simply check file system on an unmounted ext3 partition (sdb1) and try to fix errors :

This command will attempt to check /dev/sdb1, and report any errors it finds.The exit code returned by fsck is one of following conditions:

  • 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 canceled by user request

  • 128 Shared-library error

-N option just shows what would be executed but do not attempt to repair them:

-n causes these commands not to fix anything and just show what was going to be done:

Normally, fsck will skip parts of the filesystem marked as "clean" — meaning all pending writes were successfully made. The -f ("force") option specifies that fsck should check parts of the filesystem even if they are not "dirty". The result is a less efficient, but a more thorough check.

What are inodes?

As we said Linux treating everything as a file (even the hardware devices). The keyboard, mouse, printers, monitor, hard disk, processes, even the directories are treated as files in Linux. The regular files contain data such as text (text files), music, videos (multimedia files) etc. Set aside the regular data, there are some other data about these files, such as their size, ownership, permissions, timestamp etc. This meta-data about a file is managed with a data structure known as an inode (index node).

We can also check file systems using their UUID.(use blkid command ):

fsck command example

description

fsck -M /dev/sda1

prevents running fsck on mounted filesystem

fsck -t ext3 /dev/sdb1

Check Only a Specific Filesystem Type

fsck -y -f /dev/sdb1

pass “yes” to all the questions to fix

For checking a XFS filesystem, wehave to use xfs_check command

Advanced tools

There are several more advanced tools that we can use to examine or repair a filesystem.

Tools for ext2 and ext3 filesystems

  • tune2fs:Adjusts parameters on ext2 and ext3 filesystems. Use this to add a journal

  • dumpe2fs: shows all super blocks info

  • debugfs: interactive file system editor

Super Blocks

You may be wondering how all these checking and repairing tools know where to start. Linux and UNIX filesystems usually have a superblock, which describes the filesystem metadata, or data describing the filesystem itself. This is usually stored at a known location, frequently at or near the beginning of the filesystem, and replicated at other well-known locations. You can use the -n option of mke2fs to display the superblock locations for an existing filesystem.

tune2fs

The ext family of file systems also has a utility called tune2fs, which can be used to inspect information about the block count as well as information about whether the filesystem is journaled (ext3 or ext4) or not (ext2).

The tune2fs command allows you to set assorted filesystem parameters on a mounted ext2 or ext3 filesystem.

-l shows contents of the filesystem superblock, including the current values of the parameters:

The command can also be used to set many parameters or convert an ext2 filesystem to ext3 by adding a journal using -j option: tune2fs -j /dev/sdd1

Also we can use tune2fs for changing or modifying partition label:

dumpe2fs

dumpe2fs command is used to print the super block and blocks group information for the filesystem present on device.

printed information may be old or inconsistent when it is used with a mounted filesystem. Don’t forget to unmount your partition before using this command.

debugfs

Is an interactive filesystem debugger. Use it to examine or change the state of an ext2 or ext3 filesystem. It opens the filesystem in read-only mode unless we tell it not to (with -w option).

if the filesystem is mounted, is alright for inspecting, but Do not attempt repair on a mounted filesystem.

xfs_info

xfs file system has it's own family commands. xfs_info is the same as tune2fs but for xfs.

xfs_info should be used on a mounted file system.

xfsprogs package must be installed

xfs command

description

xfs_info

shows information

xfs_check

complete check of file system

xfs_repair

check and fixes problems

xfs_check -n

same as xfs_check

obviously xfs_repair does not work on mounted file system.

In XFS, you can only extend file system and can not reduce it.

Monitoring free space

On a storage device, a file or directory is contained in a collection of blocks. Information about a file is contained in an inode.

Reminder : inodes keeps information such as who the owner is, when the file was last accessed, how large it is, whether it is a directory, and who can read from or write to it. The inode number is also known as the file serial number and is unique within a particular filesystem.

Data blocks and inodes each take space on a filesystem, so we need to monitor the space usage to ensure that your filesystems have space for growth.

df

The df (DiskFree) command is used to find out about the free and used space of file systems.

If no file name is passed as an argument with df command then it shows the space available on all currently mounted file systems

-T print file system type, -h, –human-readable print sizes (in power of 1024):

-H make numbers human readable also (in powers of 1000).

-i list inode information instead of block usage:

Remember? there is no owner or access rights on vfat filesystems. vfat file format has no inodes!

df command example

description

df -a

dislpay all information includes pseudo, duplicate and inaccessible file systems.

df -Th /home

Display Information of /home File System

df -k or -m or -h

displays information in Bytes, MB , GB

df -t ext3

include specific file system type

df -x xfs

exclude specific file system type

The df command gives information about a whole filesystem. Sometimes you might want to know how much space is used by a specific file or directory, To answer this kind of question, we use the du command.

du

The Linux du (Disk Usage) command, used to check the information of disk usage of files and directories on a machine.

useful switch

description

-a

write count of all files, not just directories

-h

human readable Means we can see sizes in Bytes, KB, MB, GB,...

-c

grand total usage disk space at the last line

–max-depth=N

go N or fewer sub directories further

-s

display only total for each directory

--time option is used to display the last modification time in the output of du.

--exclude=PATTERN will exclude files that match PATTERN example: du -ah --exclude="*.txt" /home/payam

summary

Lets take a look at some other repairing tools beside tools which we have learned in this lesson:

file system

command

description

ext

tune2fs

Adjusts parameters on ext2 and ext3 filesystems and can set journaling .

ext

dumpe2fs

Prints the super block and block group descriptor information for an ext2 or ext3 filesystem.

ext

debugfs

Is an interactive filesystem debugger. Use it to examine or change the state of an ext2 or ext3 filesystem.

Reiserfs

reiserfstune

Displays and adjusts parameters on ReiserFS filesystems.

Reiserfs

debugreiserfs

Performs similar functions to dumpe2fs and debugfs for ReiserFS filesystems.

xfs

xfs_info

Displays XFS filesystem information.

xfs

xfs_growfs

Expands an XFS filesystem

xfs

xfs_admin

Changes the parameters of an XFS filesystem.

xfs

xfs_repair

Repairs an XFS filesystem when the mount checks are not sufficient to repair the system.

xfs

xfs_db

Examines or debugs an XFS filesystem.

btrfs

btrfs

Displays many aspects of btrfs filesystem information

btrfs

btrfsck

Check btrfs filesystems

btrfs

btrfs-find-root

Finds the block that is the root of the btrfs filesystem

btrfs

btrfs-debug-tree

Displays btrfs internal metadata

btrfs

btrfstune

Tune various btrfs filesystem parameters, and enables or disables some extended features

btrfs

btrfs-restore

Attempt to restore files from a damaged btrfs filesystem

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https://developer.ibm.com/tutorials/l-lpic1-104-2/

https://www.thegeekstuff.com/2012/08/fsck-command-examples/

https://www.computerhope.com/unix/fsck.htm

https://www.geeksforgeeks.org/file-system-consistency-checker-fsck/https://www.serverwatch.com/tutorials/article.php/3797306/tune2fs-Makes-It-Easy-to-Play-With-Filesystems.htmhttps://jadi.gitbooks.io/lpic1/content/1042_maintain_the_integrity_of_filesystems.html

https://linoxide.com/linux-command/linux-inode/

https://www.geeksforgeeks.org/dumpe2fs-command-in-linux-with-examples/

https://www.geeksforgeeks.org/dumpe2fs-command-in-linux-with-examples/

https://www.geeksforgeeks.org/df-command-in-linux-with-examples/

https://www.geeksforgeeks.org/df-command-linux-examples/

https://www.tecmint.com/how-to-check-disk-space-in-linux/

https://www.geeksforgeeks.org/du-command-linux/

https://www.geeksforgeeks.org/du-command-linux-examples/

https://www.tecmint.com/check-linux-disk-usage-of-files-and-directories/

With the special thanks of shawn powers.

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