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104.2 Maintain the integrity of filesystems

Iman Mahdavi Doost

8-02-2021

104.2 Maintain the integrity of filesystems

Weight: 2

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

Objectives

Verify the integrity of filesystems.
Monitor free space and inodes.
Repair simple filesystem problems.
du
df
fsck
e2fsck
mke2fs
debugfs
dumpe2fs
tune2fs
xfs tools

fsck

If anything bad happens for your filesystem (say power suddenly goes down) you will have a corrupted file system. The command to fix this is fsck. Technically this command is a front end for many commands:

jadi@funlife:~$ ls /sbin/*fsck*
/sbin/dosfsck       /sbin/fsck.ext2     /sbin/fsck.fat     /sbin/fsck.vfat
/sbin/e2fsck       /sbin/fsck.ext3     /sbin/fsck.minix
/sbin/fsck       /sbin/fsck.ext4     /sbin/fsck.msdos
/sbin/fsck.cramfs  /sbin/fsck.ext4dev  /sbin/fsck.nfs

Some of these are just hardlinks to e2fsck command

A common switch during boot is -A which tells fsck to check all file systems in /etc/fstab ordered by passno in that file which is 6th field (File systems with passno of 0, wont be checked during the boot.

root@funlife:~# fsck /dev/sdb
fsck from util-linux 2.25.1
e2fsck 1.42.10 (18-May-2014)
/dev/sdb is in use.
e2fsck: Cannot continue, aborting.


root@funlife:~# umount /dev/sdb
umount: /dev/sdb: not mounted
root@funlife:~# umount /dev/sdb1 
root@funlife:~# fsck /dev/sdb
fsck from util-linux 2.25.1
e2fsck 1.42.10 (18-May-2014)
ext2fs_open2: Bad magic number in super-block
fsck.ext2: Superblock invalid, trying backup blocks...
fsck.ext2: Bad magic number in super-block while trying to open /dev/sdb

The superblock could not be read or does not describe a valid ext2/ext3/ext4
filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>
 or
    e2fsck -b 32768 <device>

You can also check filesystems with UUID (find them with blkid command or with labels):

root@funlife:~# fsck /dev/sdb
fsck from util-linux 2.25.1
e2fsck 1.42.10 (18-May-2014)
ext2fs_open2: Bad magic number in super-block
fsck.ext2: Superblock invalid, trying backup blocks...
fsck.ext2: Bad magic number in super-block while trying to open /dev/sdb

The superblock could not be read or does not describe a valid ext2/ext3/ext4
filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>
 or
    e2fsck -b 32768 <device>

root@funlife:~# blkid 
/dev/sda1: LABEL="movies" 
/dev/sdb1: UUID="BA82-BECD" TYPE="vfat" PARTUUID="381add66-01" 
root@funlife:~# fsck LABEL=movies
fsck from util-linux 2.25.1
root@funlife:~# fsck UUID="BA82-BECD"
fsck from util-linux 2.25.1
fsck.fat 3.0.26 (2014-03-07)
/dev/sdb1: 14 files, 1972/945094 clusters

You can use -N switch to see what command/test is going to be executed:

root@funlife:~# fsck -N UUID="BA82-BECD"
fsck from util-linux 2.25.1
[/sbin/fsck.vfat (1) -- /dev/sdb1] fsck.vfat /dev/sdb1

If you want to check a XFS filesystem, you have to use xfs_check command

tune2fs

This is a command to tune ext file systems. It can show information and set many options. The -l option lists the current configs:

jadi@funlife:~$ sudo tune2fs -l /dev/sda2
tune2fs 1.42.10 (18-May-2014)
Filesystem volume name:   <none>
Last mounted on:          /
Filesystem UUID:          1651a94e-0b4e-47fb-aca0-f77e05714617
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags:         signed_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              1531904
Block count:              6123046
Reserved block count:     306152
Free blocks:              2302702
Free inodes:              1073461
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      1022
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Flex block group size:    16
Filesystem created:       Mon Dec  1 10:21:42 2014
Last mount time:          Sat Jan 31 17:21:51 2015
Last write time:          Sat Jan 31 17:21:51 2015
Mount count:              32
Maximum mount count:      -1
Last checked:             Mon Dec  1 10:21:42 2014
Check interval:           0 (<none>)
Lifetime writes:          103 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:              256
Required extra isize:     28
Desired extra isize:      28
Journal inode:            8
First orphan inode:       786620
Default directory hash:   half_md4
Directory Hash Seed:      16c38a41-e709-4e04-b1c2-8a79d71ea7e8
Journal backup:           inode blocks

xfs_info

This is same as the tune2fs but for xfs file systems.

xfs_info should be used on mounted file systems

du & df

In many cases you want to find out about the free space of a disk or find how much space a directory is using. This space can be used by the blocks of files or inodes.

inodes contain the information about files. Information like the owner, when the last time it is used or edited, its size, if its a directory or not and peoples access rights on if. The inode number is unique within a particular filesystem and is also called files serial number.

df

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

jadi@funlife:~$ df -TH
Filesystem        Type      Size  Used Avail Use% Mounted on
/dev/sda2         ext4       23G   15G  7.7G  65% /
none              tmpfs     4.0K     0  4.0K   0% /sys/fs/cgroup
udev              devtmpfs  3.9G  4.0K  3.9G   1% /dev
tmpfs             tmpfs     788M  1.4M  786M   1% /run
none              tmpfs     5.0M  4.0K  5.0M   1% /run/lock
none              tmpfs     3.9G   19M  3.9G   1% /run/shm
none              tmpfs     100M   28K  100M   1% /run/user
/dev/mapper/chome ext4      243G  229G   14G  95% /home/jadi
/dev/sdb1         vfat      3.7G  7.8M  3.6G   1% /media/jadi/BA82-BECD

Here the `-T` switch make df to show the file system types and `-H` make numbers human readable (in powers of 1000). Please note that `-h` is also human readable but in powres of 1024 (e.g. shows 1k for 1000 bytes.

If you need the inode data, use the -i switch:

jadi@funlife:~$ df -i
Filesystem          Inodes  IUsed    IFree IUse% Mounted on
/dev/sda2          1531904 458616  1073288   30% /
none               1007533      4  1007529    1% /sys/fs/cgroup
udev               1003703    542  1003161    1% /dev
tmpfs              1007533    644  1006889    1% /run
none               1007533      3  1007530    1% /run/lock
none               1007533    162  1007371    1% /run/shm
none               1007533     33  1007500    1% /run/user
/dev/mapper/chome 16171008 269293 15901715    2% /home/jadi
/dev/sdb1                0      0        0     - /media/jadi/BA82-BECD

vfat file format has no inodes; there is no owner or access rights on vfat filesystems.

du

The DiskUsage command give information about the used space of directories and files. The common switches are:

switch	usage
-h	print sizes in powers of 1024 (e.g., 1023M)
-H	print sizes in powers of 1000 (e.g., 1.1G)
-c	show the grand total
--max-depth 2	shows only 2 directories furthur
-s	Only shows the summary and not all the directories one by one

jadi@funlife:~/w/lpic$ du 
16    ./101
701456    ./done
701464    ./Logo/chert
704588    ./Logo
12    ./data
12    ./100
9432884    .
jadi@funlife:~/w/lpic$ du -c
16    ./101
701456    ./done
701464    ./Logo/chert
704588    ./Logo
12    ./data
12    ./100
9432884    .
9432884    total
jadi@funlife:~/w/lpic$ du -hs
9.0G    .

Repairing

We used the fcsk for showing file system information but if is designed to fix file systems too. If the boot time check find a problems, you will be put into a command line to fix the problems.

On non-journaling file systems (ext2) the fsck will show you many questions about each block and you have to say y if you want it to fix them. On journaling file systems (ext3&4, xfs, ..) the fsck has much less tasks to perform.

for xfs file systems, we have xfs_check command

An important switch is -n which causes these commands not to fix anything and just show what was going to be done.

debugfs

This is an interactive tool for debug an ext filesystem. It opens the filesystem in read-only mode unless we tell it not to (with -w option). It can un-delete files and directories..

root@funlife:~# debugfs /dev/sda2
debugfs 1.42.10 (18-May-2014)
debugfs:  cd /etc/        <-- cd 
debugfs:  pwd            <-- show were am I
[pwd]   INODE: 524289  PATH: /etc
[root]  INODE:      2  PATH: /
debugfs:  stat passwd        <-- show data on one file
Inode: 527187   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 1875144872    Version: 0x00000000:00000001
User:     0   Group:     0   Size: 2145
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 8
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x548d4241:a7b196fc -- Sun Dec 14 11:24:41 2014
 atime: 0x54cc635b:6acfc148 -- Sat Jan 31 08:38:43 2015
 mtime: 0x548d4241:a01076f8 -- Sun Dec 14 11:24:41 2014
crtime: 0x548d4241:9f1c52f8 -- Sun Dec 14 11:24:41 2014
Size of extra inode fields: 28
EXTENTS:
(0):2188172
debugfs:  ncheck 527187        <-- node check an inode
Inode    Pathname        
527187    /etc/passwd
debugfs:  q            <-- quit

Superblock

Unix systems use superblocks to save filesystem metadata. Most of the times this block is located at the beginning of the file system and replicated on other locations too. The -n of mke2fs displays superblock locations

# mke2fs -n /dev/sda7
mke2fs 1.41.9 (22-Aug-2009)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
7159808 inodes, 28637862 blocks
1431893 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
874 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks: 
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
    4096000, 7962624, 11239424, 20480000, 23887872

Other tools

For the LPIC exam, it is good to know about these commands.

filesystem	command	usage
ext	tune2fs	Show or set ext2 and ext3 parameters or even set the journaling options
ext	dumpe2fs	Prints the super block and block group descriptor information for an ext2 or ext3 filesystem.
ext	debugfs	Is an interactive file system debugger. Use it to examine or change the state of an ext2 or ext3file system.
reiserfs	reiserfstune	show and set parameters
reiserfs	debugreiserfs	Prints the super block and block group descriptor information for an ext2 or ext3 filesystem.
XFS	xfs_info	display information
XFS	xfs_growfs	expand file system
XFS	xfs_admin	change parameters on XFS file systems
XFS	xfs_repair	repair the problems
XFS	xfs_db	checks and debugs the filesystem