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How much disk space does a new empty file take?

2020-11-06 21:04:16 Zhang Yanfei Allen

Today, let's think about a simple question . stay Linux You can use touch Command to create an empty file :

touch empty_file.txt

After the operation is completed , Whether to consume some of our disk space ? If necessary , About how much ? Um. , Yes , This question is more simple than you think , But I don't know if you can give yourself a satisfactory answer .

My previous articles are all about the physical layer of disk , But it may not be enough to help understand the problems associated with the document . Starting today, let's move up the physical plane , To Linux Find the answer in the file system principle .

True knowledge comes from practice

I think it's possible to abandon the kernel principle first , It's more interesting to experiment directly by hand . You must know ls This command allows you to view the file size , So let's take a look at it .

# touch abcdefghigklmn.txt  
# ls -l  
total 0  
-rw-r--r-- 1 root root 0 Aug 17 17:49 empty.file  

forehead ,ls The command tells me that this empty file takes up 0. The size of the file is really 0, Because we haven't written anything to the file yet . But what we have to think about now is , Whether an empty file takes up disk space . So intuition tells us it's absolutely impossible , There is an extra file on the disk , How can it be that there is no space cost at all !

To solve this mystery , You need help df command . Input df –i

# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
......
/dev/sdb1            2147361984 12785019 2134576965    1% /search

This output helps us to show what's going on in our file system inode Usage situation . Be careful IUsed yes 12785019. We continue to create an empty file

# touch empty_file2.txt
df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
......
/dev/sdb1            2147361984 12785020 2134576964    1% /search
[@bjzw_46_76 temp]#

Now pay attention to IUsed Turned into 12785020.

ha-ha , One of our conclusions is that . Creating an empty file will take up a Inode.

Elaborate inode

that inode What kind of file related information is stored in it ? Let's take a look at the source code of the kernel . You can download one linux Source code . With ext2 File systems, for example , In my download of linux-2.6 Files in fs/ext2/ext2.h in , You can find the kernel for inode Definition of structure . The structure is more complex , It mainly stores some other data besides the contents of the file , Let's pick some of the more critical intercepts :

struct ext2_inode {
        __le16  i_mode;         #  File permissions 
        __le16  i_uid;          #  File owner ID
        __le32  i_size;         #  File Bytes size 
        __le32  i_atime;        #  The last time the file was accessed 
        __le32  i_ctime;        #  File creation time 
        __le32  i_mtime;        #  When the document was modified 
        __le32  i_dtime;        #  When the file was deleted 
        __le16  i_gid;          #  File group ID
        __le16  i_links_count;  #  This document inode The number of times it was connected 
        __le32  i_blocks;       #  Of documents block Number 
		......
        __le32  i_block[EXT2_N_BLOCKS]; #  An array that points to blocks that store file data 
		......

You can see the user associated with the file 、 Visit time and so on exist inode Medium . In addition to include/linux/fs.h in , There's another. VFS Level of inode The definition of , We don't diverge here . Use stat Command to see the file directly inode Data in the .

# stat test
  File: `test'
  Size: 0               Blocks: 0          IO Block: 1024   regular empty file
Device: 801h/2049d      Inode: 26          Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-03-01 12:14:31.000000000 +0800
Modify: 2020-03-01 12:14:31.000000000 +0800
Change: 2020-03-01 12:14:31.000000000 +0800

Every inode How big is it ?dumpe2fs I can tell you (XFS Use xfs_info).

# dumpe2fs -h /dev/mapper/vgroot-lvroot
dumpe2fs 1.41.12 (17-May-2010)
......
Inode size:               256

Inode size Represent each Inode Size . On my machine , Every inode All are 256 byte . Two inode Is exactly the size of the disk sector 512 byte .

Where is the file name

inode We've seen all the structures , After working for a long time, I don't know if I have found a problem ,inode There is no file name stored in it !! that , Where is the file name ?

stay fs/ext2/ext2.h in , I found the following folder related structures

struct ext2_dir_entry {
         __le32  inode;                  /* Inode number */
         __le16  rec_len;                /* Directory entry length */
         __le16  name_len;               /* Name length */
         char    name[];                 /* File name, up to EXT2_NAME_LEN */
};

This structure is our usual folder . you 're right , The file name exists in the folder data structure to which it belongs , It's one of them char name[] Field . Along with the file name , The files in the folder were also recorded inode Etc .

Conclusion

    1. A new empty file needs to be consumed inode, To save users 、 Creation time and other metadata .
    1. To create an empty file, you need to consume all of its directories block A certain amount of space in , This space is used to hold the file name , jurisdiction 、 Time and other information

therefore , It looks like a new empty file , As long as you want to dig , Can really dig out a lot of knowledge . Finally, I'd like to share a fault encountered by my classmates in our team . One of our offline machines has stopped cooking directly , After restart, the reason is inode It's consumed . Tracing again found that a process created too many empty log files . Even though the files are empty , however inode It's wasted . Later, the students in charge changed the logic of creating log files , Deleted the extra empty files , The machine is back to normal .


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