Hierarchical File System (HFS)

Written by John Harris, Updated on October 10, 2023

Hierarchical File System (HFS) is the file system used by the Mac Operating System and developed by Apple Inc. In the beginning, HFS was designed to work on floppy, CD-ROMs, and hard disk drives. HFS is referred to as a standard for Mac Operating Systems and the successor of HFS i.e. HFS Plus is also known as HFS Extended.

Components of Macintosh File

File stored on Macintosh Operating System contains two forks:

  • Resource Fork: This component is an indexed file which contains code segments, dialog boxes, menu items etc.
  • Data Fork: This component contains a stream of bytes.

Each and every fork in a file comprises one or more extents of blocks, which encode the extent’s starting block and the size. A file’s catalog record contains a file’s first extent of a fork. The further extents getting stored on the Mac are saved in the form of a B-Tree.

Each file stored on the HFS file system has an identification number. A file is named based on its parent’s identification number and the directory’s file name, as it is a 32-character string that also contains nulls.

The new catalog B-Tree is different from the traditional B-Tree structure, as all the nodes at each level are linked together to form a doubly linked list, with the records at the leaf nodes.


The technique that is used to protect the integration of Mac Operating System Extended file systems on Mac Operating System X volumes is known as Journaling. When the journaling process is enabled, it keeps track of and records each and every functionality going on in the system.

If your system is functioning well due to data loss scenarios like power fluctuations or virus attacks, the journal file will help you restore it back to the same state after restarting the server.

Journaling is the most important and best feature to use for servers with many files and high availability

Note: The HFS file system was replaced by HFS+ in 1998 which was once again replaced by APFS (Apple File System) in 2019 with macOS 10.15

Related Information

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