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How much disk space does a file of 1 byte actually occupy

2020-11-06 21:04:14 Zhang Yanfei Allen

In the foreword 《 Whether a new empty file takes up disk space ? How much does it take 》 We learned about the disk overhead of an empty file . Today, let's think about another question , If we only write 1 Bytes , So the actual disk usage of this file is also 1 Bytes ?

see 1 Byte file

As before , Let's not talk about the principle , Just do it yourself .

# mkdir tempDir
# cd tempDir
# du -h
0    .
# touch test
# du -h
0    .

After creating an empty file in a directory , adopt du The footprint of the folder as seen by the command has not changed . This is in line with our previous understanding , Because empty files only occupy inode. good , Let's modify the file , Add a letter

echo "a" > test
# du -h
4.0K    .

After saving, check the space usage of the directory again . We found that the original 0 Increased to 4K. So , No matter how small the contents of the document , Even a byte , In fact, the operating system will also assign you 4K Of . Oh , Of course, we have to calculate the above mentioned inode And file names stored in the folder data structure . therefore , Don't maintain a lot of broken files in your system . File size , It takes up a lot of disks !

Notice that my experimental environment is in ext Under the file system . If it is xfs There may be some discrepancy in performance .

Keep talking about this 4K

And then linux Source code file fs/ext2/ext2.h About inode The definition of , We find the data node defined in the structure block Array :

struct ext2_inode {
        ......
        __le32  i_block[EXT2_N_BLOCKS]; #  An array that points to blocks that store file data 
        ......

When a file has no data to store , This array is empty . And when we write 1 A few bytes later , The file system needs to apply for block To store , After the application , The pointer is in this array . Even if the file contains only one byte , It will still be assigned a whole Block, Because this is the smallest working unit of a file system . So this block How big is it ,ext You can go through dumpe2fs see .

#dumpe2fs -h /dev/mapper/vgroot-lvroot
......
Block size:               4096

It's on my machine , One Block yes 4KB.

What to do if the content of the document is too large

I don't know if you pay attention to ,inode As defined in block How about the array size , Only EXT2_N_BLOCKS individual . Let's look at the definition of this constant again , Find out it's 15, The definition in the relevant kernel is as follows :

#define EXT2_NDIR_BLOCKS                12
#define EXT2_IND_BLOCK                  EXT2_NDIR_BLOCKS
#define EXT2_DIND_BLOCK                 (EXT2_IND_BLOCK + 1)
#define EXT2_TIND_BLOCK                 (EXT2_DIND_BLOCK + 1)
#define EXT2_N_BLOCKS                   (EXT2_TIND_BLOCK + 1)

Just press the 4K Of block size Look at ,15 individual block Just enough to save 15*4=60K The file of . I believe you are not satisfied with the size of this file , You save one avi Big movies have to be on G 了 . that Linux How to achieve large file storage ? Um. , In fact, the definition process of macro above has already told you , It's just 12 Arrays are stored directly block The pointer , The rest is used for indirect indexing (EXT2_IND_BLOCK), Secondary indirect index (EXT2_DIND_BLOCK) And the third level index (EXT2_TIND_BLOCK).

file

such , The space that a file can use expands exponentially . When the files were small , They all use direct index , disk IO Less , Good performance . When the files are big , Visit one block It might have to be done three times first IO, Performance is a little bit slow , However, there are OS Page caching at level 、 Directory item cache bonus , It's OK .

Conclusion

File systems are managed in blocks , So no matter how small your file is , Even if it's only one byte , Will consume a whole block . This block size can be passed through dumpe2fs Wait for the order to see . What if you want to change the size of this block ? I'm sorry , You can only reformat .


file


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