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Ext4 file system opens dir_ After nlink feature, link_ Use link after count exceeds 65000_ Count=1 means the quantity is unknown

2022-07-26 09:17:00 【Wangwanlin Ben】

As shown in the figure below , Create a tempdir Folder , Then create subfolders in batch ：

First create 64998 Subfolders , You can see link_count from 2 Add to 65000 individual , In line with expectations ;

To create a 1 A folder , see link_count It turned out to be 1; And delete some subfolders , Stay in 64998 Within a ,link_count Nor did it return to the correct value .

View file system types , yes ext4

principle

Ext4 Disk Layout - Ext4

0x1A

__le16

i_links_count

Hard link count. Normally, ext4 does not permit an inode to have more than 65,000 hard links. This applies to files as well as directories, which means that there cannot be more than 64,998 subdirectories in a directory (each subdirectory's '..' entry counts as a hard link, as does the '.' entry in the directory itself). With the DIR_NLINK feature enabled, ext4 supports more than 64,998 subdirectories by setting this field to 1 to indicate that the number of hard links is not known.

Reference material

Ext4 Disk Layout - Ext4

196405 – mkdir mishandles st_nlink in ext4 directory with 64997 subdirectories

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