What are the ONTAP limitations on files, directories, and subdirectories?

Limits encountered on creating a file or a directory:

• Maximum size of the parent directory (that is, the size of the directory file. See man pages for -maxdirsize-):
  An error message is generated when this limit is reached and an error is returned to the client, which generally interprets the error as a full volume, although this does not necessarily mean that the volume is out of space.
• The number of sub-directories:
  • The link count field has 4 bytes, which allows for approximately 4.3 billion links/subdirectories.
  • This number might not be changed. To understand the reasons for this limit, see the section below on hard links and subdirectory implementation.
    • Number of files that can exist in a volume (that is, the size of the inode file. See man pages for -maxfiles-).
• Maximum size of the volume (see man pages for -vol size-).
• Quotas implemented on qtrees can limit the maximum space a client might use (see man pages for -qtree-).

Information on directories:

• The purpose of a directory is to provide a mapping between an inode and one or more names, possibly with different paths.
• A directory is nothing else than a 'special' file that uses up an inode.
• The directory contains the inode number and one or more filenames in different formats (Unicode, NFS, 8.3).
• The number of files that can be stored in any one directory very much depends on the length of the file name and the type of filename used.
• Unicode names usually need more space than NFS names.
• The largest amount of space is used when names are stored in Unicode, NFS, and 8.3 at the same time (see also man pages for -vol options  create_ucode | convert_ucode- ).
• The size of the directory file is controlled by -vol options maxdirsize-, but be aware that directories can never shrink.

Information on inodes and the inode file:

• Inode information is held in the inode file, which is a hidden system file, and each inode is 288 bytes long (192 bytes on Data ONTAP releases earlier than 9.0).
• The inode file normally cannot shrink, so be cautious when using the -maxfiles- command.
  • The maximum is one inode per WAFL block (4 kb).
  • The minimum and default value is one inode per 32 kb of volume space.
• You can decrease the value of the maxfiles parameter for a volume, as long as you do not decrease it below the current number of files stored in the volume.

Additional information on hard links and subdirectories:

• A hard link is a mapping between a name (or a unified group of names) and an inode number and is held in a directory.
• An inode number can be referenced by multiple names, and each reference adds to the link count for the inode.
• Directories make extensive use of and are implemented using hard links.
• While ONTAP has a limit of 100.000 hardlinks per inode, it does not apply to directories.
• Hard links are used to show connectivity between parent and child directories in a directory tree. 
• Each directory has a minimum of two entries '.' and '..'.  Where '.' is a hard link to itself and '..' is a hard link to the parent directory.
• Therefore, every subdirectory created adds to the hard link count of the parent directory leading to the subdirectory limits described above.
• Highly schematic view of a file system structure (the actual implementation looks different but the view is good for a general understanding):