In-depth analysis of file system principles (inode, difference between soft and hard links)

SuperBlock stores all Inode and block related information of the file system.

When a user tries to access a file in the Linux system, the system will first search its inode based on the file name to see if the user has the permission to access the file. If yes, point to the corresponding data block; if not, return Permission denied (access denied)

(4) Delete the specified The file corresponding to the inode number

Format: find ./ -inum inode number -exec rm -i {} \;

Example:When the file name contains special symbols, we can delete the inode number to delete the file.

[root@localhost ~]# touch ‘a b`_c de ! 4|’.txt

[root@localhost ~]# ls -li

[root@localhost ~]# find ./ -inum 34097384 -exec rm -i {} \;

rm: Do you want to delete the ordinary empty file “./a b`_c de ! 4|.txt”? y

[root@localhost ~]# ls

Look for files modified within 3 days from the /root/ directory and copy them to /tmp

[root@localhost ~]# ls /tmp/

[root@agent ~]# find /root/ -mtime -3 -a -type f -exec cp {} /tmp \;

(5) View File system inode and block information

df -i device name (query when the file system is mounted, query the total number of inodes and the number used)

dumpe2fs -h device name (the file system does not need to be mounted)

tune2fs -l device name (the file system does not need to be mounted)

Example:

[root@localhost ~]# df -i /data

[root@localhost ~]# umount /data

[root@localhost ~]# dumpe2fs -h /dev/sdb1

[root@localhost ~]# tune2fs -l /dev/sdb1

(6) Specify the number of inodes and block size of the file system when formatting

mkfs.ext4 -N inode number -b block size (in bytes) device name

Example:The number of inodes is set to 60,000, and the block size is 1KB

[root@localhost ~]# mkfs.ext4 -N 60000 -b 1024 /dev/sdb1

[root@localhost ~]# dumpe2fs -h /dev/sdb1 | grep -i “inode count”

[root@localhost ~]# dumpe2fs -h /dev/sdb1 | grep -i “block size”

(7) Solve the inode Disk failure due to exhaustion

1. Delete unused files and configure disk quotas

[root@localhost ~]# find /data -mtime -1 -exec rm -rf {} \;

[root@localhost ~]# rm -rf `find /data -mtime -1` “ = $()

[root@localhost ~]# find /data -mtime -1 |xargs rm -rf

2． Back up the file, reformat the file system, and specify a larger number of inodes

[root@localhost ~]# df -i /test

[root@localhost ~]# touch /test/{1..28213}.txt

[root@localhost ~]# touch /test/hello.txt

Use the second method:

[root@localhost ~]# mkdir /backup

[root@localhost ~]# mv /test/* /backup/

[root@localhost ~]# umount /test

[root@localhost ~]# mkfs.ext4 -N 50000 /dev/sdb2 &> /dev/null

[root@localhost ~]# mount /dev/sdb2 /test

[root@localhost ~]# mv /backup/* /test

mv: overwrite `/test/lost + found’? y

[root@localhost ~]# touch /test/hello.txt

4. Principles of soft and hard links in Linux file system

(1) Hard link (hard link)

Method: ln source file target file

Features:

The inode number of the file pointed to by the hard link. The inode number of the newly generated hard link file is the same as the inode number of the source file. Hard links cannot be made to directories and must be in the same file system. Deleting one file name does not affect access to the other.

Example:

[root@localhost ~]# df -h /data

[root@localhost ~]# cd /data

[root@localhost data]# ln a.txt hardlink-a.txt

[root@localhost data]# ls -li

Delete the source file, hard links will not be affected

[root@localhost data]# echo “abc” > a.txt

[root@localhost data]# rm -rf a.txt

[root@localhost data]# cat hardlink-a.txt

(2) Soft link (soft link)

Method: ln -s source file or directory target file or directory

Features:

Soft links are also called symbolic links

The file name pointed to by the soft link. The inode number of the newly generated soft link file is different from the source file. The directory can also generate soft links. The soft link file and the source file do not need to be in the same file system. The content of the soft link file belongs to the source file. Path, when reading, the system will automatically direct to the source file path and find the file content based on the source file. However, when the source file is moved or renamed, the soft link will report an error.

Example:

[root@localhost data]# echo "soft test" > a.txt

[root@localhost data]# ln -s a.txt softlink-a.txt

[root@localhost data]# ll -i

Rename the source file, the hard link will not be affected, but the soft link will become invalid

[root@huyang1 data]# ls -li

[root@huyang1 data]# ln a.txt hardlink.txt

[root@huyang1 data]# ls -li

[root@localhost data]# mv a.txt b.txt

[root@localhost data]# ls -li

[root@huyang1 data]# cat b.txt

[root@huyang1 data]# cat hardlink.txt

[root@localhost data]# cat softlink-a.txt 

(3) Hard links and Comparison of soft links