Linux file system operation implementation

This reading note mainly records the operations related to the file system.

Disk and directory capacity

The overall data of the disk is in the superblock, but the capacity of each individual file is recorded in the inode. Two commands are commonly used to display disk usage:

df: List the overall disk usage of the file system

du: Evaluate the disk usage of the file system (often used to evaluate the capacity occupied by the directory)

$ df [-ahikHTm] [directory or file name]
parameter:
-a: List all file systems, including system-specific /proc (/proc is mounted in memory and does not occupy disk space) and other file systems;
-k: Display the capacity of each file system in KB -m: Display the capacity of each file system in MB -h: Display in GB, MB, KB and other formats that are easier for people to read -H: Replace M=1024K with M=1000K -T: List the file system name of the partition (such as ext3)
-i: Display in inodes instead of disk capacity

The data that df mainly reads is almost all for the entire file system, so the reading range is only the information within the super block, so this command displays the results very quickly.

$ du [-ahskm] File or directory name Parameters:
-a lists all file and directory capacities, because by default only the file capacities of directories are counted -h displays the capacity in human-readable form -s lists the total amount, without listing the capacity occupied by each individual directory -S does not include the total under subdirectories -k lists the capacity in KB -m lists the capacity in MB

du will directly search all file data in the file system

Link file: ln

There are two types of connection files under Linux: one is similar to the shortcut on Windows, which allows you to quickly connect to the target file or directory. The other is to generate a new file name through the inode connection of the file system instead of generating a new file. This is called a hard link.

hard link (hard link and actual link)

• Each file occupies an inode, and the contents of the file are pointed to by the inode record.
• If you want to read a file, you must use the file name recorded in the directory to point to the correct inode number before you can read it.
• The file name is related to the directory, and the file content is related to the inode. A hard link is an association record that creates a new file name in a directory and connects to a certain inode number.

vagrant@vagrant-ubuntu-trusty-64:~$ cd /tmp
vagrant@vagrant-ubuntu-trusty-64:/tmp$ touch tes
vagrant@vagrant-ubuntu-trusty-64:/tmp$ ln test test1
vagrant@vagrant-ubuntu-trusty-64:/tmp$ ll -i test test1
62273 -rw-rw-r-- 2 vagrant vagrant 0 Dec 17 12:39 test
62273 -rw-rw-r-- 2 vagrant vagrant 0 Dec 17 12:39 test1

It can be seen that the inode numbers of the two files are the same, their file permission attributes are exactly the same, and the number of connections has become 2.

The biggest advantage of hard links is security. If you delete any file name, the inode and block still exist. At this time, you can read the correct file data through another file name. No matter which file name you use to edit, the final result will be written to the same inode and block, so the data can be modified.

Generally speaking, when you use hard link to connect files, the disk space and the number of inodes will not change. Hard link just writes one more associated data in a block under a directory. It will neither increase the inode nor consume the number of blocks (unless the disk is full when you add one more associated data to the block, in which case you may need to add one more block to record the file name association, which will cause a change in disk space).

Limitations of hard links:

• Cannot cross file systems
• You cannot connect to a directory because if you use a hard link to connect to a directory, the connected data needs to be connected together with all the data under the connected directory, which will cause considerable complexity in the environment. Therefore, directories are not supported for the time being.

symbolic link (a symbolic link, also known as a shortcut)

A symbolic link creates an independent file, and this file will point the data reading to the file name of the file it links to. Since it only uses the file as a pointing operation, when the source file is deleted, the symbolic link file will not be able to find the source file and cannot be opened.

vagrant@vagrant-ubuntu-trusty-64:/tmp$ ln -s test test2
vagrant@vagrant-ubuntu-trusty-64:/tmp$ ll -i test test2
62273 -rw-rw-r-- 2 vagrant vagrant 0 Dec 17 12:39 test
62275 lrwxrwxrwx 1 vagrant vagrant 4 Dec 17 13:07 test2 -> test

The two files point to different inode numbers. The important thing about the connected file is that it will write the file name of the target file. Because the file on the right side of the arrow is 4 bytes, the size of the connected file is 4 bytes.

The file created by the symbolic link is an independent new file, so it will occupy the inode and block.

When you modify a symbolic link file, the source file will be modified.

$ ln [-sf] Source file target file parameters:
-s: If you connect without any parameters, it is a hard link, and -s is a symbolic link
-f: If the target file exists, delete it and rebuild it.

Number of links to the directory

When a file is connected with a hard link, the number of file connections will increase by 1. When we create an empty directory, since there are two directories, . and .., when we create an empty directory /tmp/testing, there will basically be three things:
/tmp/testing
/tmp/testing/.
/tmp/testing/..
/tmp/testing and /tmp/testing/. both represent the directory, and /tmp/testing/.. represents the directory /tmp. So when we create a new directory, the number of links to the new directory is 2, and the number of links to the parent directory increases by 1.

Disk partitioning, formatting, checking and mounting

If you want to add a new hard disk to the system, you need to do the following:

1. Partition the disk to create new available partitions
2. Format the partition to create a file system that can be used by the system.
3. If you want to be more careful, you can check the newly created file system.
4. On a Linux system, you need to create a mount point (that is, a directory) and mount it.

Disk partition: fdisk

$ fdisk [-l] Device name parameter:
-l: Output the contents of all partitions of the device that follows. If only fdisk -f is used, the system will list all the partitions of the devices that the entire system can find.

# For example:
# First find the disk file name vagrant@vagrant-ubuntu-trusty-64:/tmp$ df /
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/hdc1 41251136 3631948 35883276 10% /
# Type fdisk without the number vagrant@vagrant-ubuntu-trusty-64:/tmp$ fdisk /dev/hdc
It will prompt Command (m for help): Enter m to see the relevant parameter prompts d means to delete a partition n means to add a partition p means to display the partition table on the screen q means to exit the fdisk program without saving w means to write the previous operation to the partition table

Pay special attention to q and w. If you press q when leaving fdisk, all operations will not take effect. On the contrary, pressing w means that the operations will take effect.

Deleting a disk partition

To delete a partition (for example, to delete all partitions of /dev/hdc), you need to do the following steps:

1. fdisk /dev/hdc: first enter the fdisk interface
2. p: First look at the partition information, assuming you want to delete /dev/hdc1
3. d: At this time, you need to select a partition, so select 1
4. w or q: w can store to the disk data table and exit fdisk; if you regret it, just press q to cancel the deletion operation.

Add a new disk partition

The following actions need to be performed:

1. fdisk /dev/hdc: first enter the fdisk interface
2. n: Add a new partition
3. p or e or l: Select different partition types, where p stands for primary partition, e stands for extended partition, and l stands for logical partition
4. 1 - 4: Partition number, you can choose 1-4. If it is a logical partition, you do not need to enter the partition number
5. Enter the ending cylinder number. If it is too troublesome to calculate the cylinder/partition size yourself, you can use a format like "+512M" to let the system help us allocate the cylinder number closest to 512M.
6. p: View partition information
7. w or q: w can store to the disk data table and exit fdisk; if you regret it, just press q to cancel the deletion operation.

Regarding the form of creating partitions (primary partition/extended partition/logical partition) and the size of the partition, generally speaking, there are several forms of creating new partitions:

1. There are still 1-4 remaining, and the system has no extended partition: At this time, an option will appear for you to select Primary/Extended, and you can specify a number between 1 and 4
2. There are still 1-4 partitions left, and the system has an extended partition: You will be asked to select Primary/Logical. If you select p, you need to specify a number between 1 and 4. If you select l, you do not need to set a number because the system will automatically assign a logical partition file name number.
3. 1-4 is not left, and the system has an extended partition: You will not be asked to select the partition type at this time, and will directly enter the logical partition mode

Generally, after partitioning, you need to reboot to update the kernel partition table information. You can use "partprobe" to force the kernel to find the partition table again.

The above is the full content of this article. I hope it will be helpful for everyone’s study. I also hope that everyone will support 123WORDPRESS.COM.