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$ ls -l

total 28

drwxrwxrwx 1 t t 4096 May  8  2018  dir1

drwxrwxrwx 1 t t    0 Mar 12  2015  dir2

drwxrwxrwx 1 t t 4096 Jan 17  2014  dir3

drwxrwxrwx 1 t t 4096 Jun 12  2017  dir4

drwxrwxrwx 1 t t    0 Aug 24  2012  dir5

Why is the size of a directory either 0 or 4096?

Is the size of a directory 0 because the entries in it can fit into its inode?

Is the size of a directory not 0 because the entries in it can't fit into its inode and thus need some data block(s)?

Why is the nonzero size of a directory always 4096?

Thanks.

Note my observation is the same in ext4 filesystem and in NTFS filesystem. Does it mean both filesystem types implement directories in a similar way?

Some filesystems have some space in the inode that can be used to hold very small directories. But once the directory is large enough to not fit in that area, the OS will allocate an entire file allocation unit, which in your case is apparently 4096 bytes.

On ext4, this is enabled with the inline_data ext4 option.

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Inline_Directories

About 4096

To quote Terry Wang's answer:

A directory is just a special file which contains an array of filenames and inode numbers. When the directory was created, the file system allocated 1 inode to the directory with a "filename" (dir name in fact). The inode points to a single data block (minimum overhead), which is 4096 bytes.

Directories can grow larger than 4096, and one of the peculiar "bugs" of ext4 filesystem which did not receive much priority yet is that after directory information has outgrown 4096 bytes, that number is not decremented if files are removed. And probably the big news flash is that how information is allocated for a directory depends on the filesystem and even the options which are enabled/disabled( source ), so the special number 4096 is specific to ext4 in this case, but could be something else on other filesystems such as UFS for example.

_{Note, however, Terry's talking about inode which represents directory, directory itself being a dirent in parent directory. Initial directory contents are dirents or links . and ..}

About 0

The ext4 filesystem defaults to creating directories ( even if empty ) with 4096 bytes allocation. That's the key. According to ServerFault and Arch Linux forum when directory shows as 0 in size it means the directory is located or hosts a filesystem other than ext4.

And this is true of virtual filesystems (which effectively aren't physical on-disk filesystems, but rather are an interface exposed by kernel and are contained in memory):

$ ls -ld /proc /sys 

dr-xr-xr-x 243 root root 0 Feb 24 13:57 /proc

dr-xr-xr-x  13 root root 0 Feb 26 14:42 /sys