What Is HFS+?

File systems organize how data is stored and retrieved on storage devices. HFS+ is one example of a file system that manages how information is arranged and accessed, specifically within Apple computing environments. It was introduced to address limitations found in older file system formats and incorporates enhanced features for handling large volumes of data.

HFS+ stands for Hierarchical File System Plus. It is an enhanced version of the original Hierarchical File System (HFS) designed by Apple. HFS+ was introduced in 1998 alongside Mac OS 8.1 as a more robust solution for hard disk organization, improved allocation of large storage volumes, and more efficient file management. It uses a B-tree-based data structure for directories and file metadata. The system also integrates extended attributes, Unicode file naming, and journaling capabilities to reduce corruption risks during unexpected shutdowns or crashes.

What Is HFS+ Used For?

HFS+ is known for handling larger file sizes compared to its predecessor, supporting efficient data retrieval, and maintaining file integrity through journaling.

Here are the primary purposes of HFS+:

Managing disk partitions on older and legacy Mac computers.
Storing and retrieving large files in macOS environments (especially prior to the introduction of Apple File System (APFS).
Providing backward compatibility with older Mac operating systems.
Offering a journaling feature to maintain consistency in case of power failures or unexpected restarts.

Which OS Uses the File System HFS+?

HFS+ has been tied to Apple’s operating systems for many years. Earlier versions of macOS and certain Apple-related systems relied heavily on HFS+ before APFS became the default.

Here is a list of operating systems that have implemented HFS+:

Mac OS 8.1 through Mac OS 9.
macOS (formerly OS X) versions up to macOS High Sierra (partial transition to APFS started around macOS Sierra for solid-state drives).
macOS recovery partitions on older macOS releases.
iPod software (classical iPod models used HFS+ when mounted on macOS).

HFS+ Structure

Here are the key elements of HFS+ structure:

Volume header. Holds fundamental details about the volume, such as the block size, the total number of files, and references to other critical structures. It serves as the primary entry point for system utilities that analyze or repair the file system.
Allocation file. Identifies which blocks on the storage medium are already occupied and which are available for new data. It functions like a map for free and in-use disk space, enabling the system to locate suitable blocks for file growth and new file creation.
Catalog file. Implements a B-tree that records file and folder information. Entries include file names, file IDs, and their hierarchical relationships. The use of a B-tree structure speeds up searching, insertion, and deletion tasks, which is crucial for maintaining efficient performance on larger volumes.
Extents overflow file. Stores detailed information about file extents when a file becomes too large or highly fragmented to be described fully within the main catalog. Since these overflow records are also organized via a B-tree, lookup times remain predictable and manageable.
Attributes file. Retains extended metadata attributes associated with files, such as application-specific data, custom icons, and user-defined tags. By isolating extended attributes in a dedicated file, HFS+ preserves a clean separation between core data and supplementary metadata.
Startup file. Assists with boot processes under certain legacy Mac configurations. It can contain code or system-specific information required to start older Mac operating systems or enhance compatibility with special hardware.

Features of HFS+

HFS+ incorporates multiple technical components that enhance efficiency, data integrity, and compatibility within Apple-centric ecosystems. Here are its main features:

Journaling

Journaling in HFS+ tracks changes to the file system’s metadata, such as directory updates and file allocation information. The journal is stored as a continuous log of metadata operations, including file creations, deletions, and renames.

Journaling ensures consistency by either replaying or discarding incomplete transactions if a system shutdown, kernel panic, or power loss occurs. The journal itself is maintained in a dedicated area on the storage device, allowing HFS+ to validate the integrity of file system structures when the system restarts.

This journaling feature aids in reducing disk corruption and simplifies the process of file system recovery because it alleviates the need for time-consuming scans of the entire disk.

Unicode Support

HFS+ supports Unicode-based file names, allowing a broad range of characters from numerous writing systems. This Unicode support accommodates languages using non-Latin scripts, such as Chinese, Japanese, Arabic, and Cyrillic, as well as extended symbols and diacritic marks.

The adoption of Unicode-based file names removes many of the naming restrictions seen in older file systems, enabling multilingual naming conventions and special characters. Unicode compliance ensures that files retain consistent naming across different software environments that rely on Unicode character handling.

Large File and Volume Support

HFS+ provides the ability to handle large file sizes and disk volumes more effectively than its predecessor, HFS. This improvement is attributed to the use of 32-bit allocation mapping, which offers broader capacity than the original 16-bit approach used by HFS.

The theoretical maximum volume size of HFS+ reaches multiple terabytes, ensuring modern storage devices can be fully utilized without hitting file system-imposed limits. Efficient block allocation strategies further enhance performance by decreasing the likelihood of early file fragmentation. Large file handling allows HFS+ to store high-resolution media, complex software libraries, and substantial data archives without requiring additional partitioning.

Extended Attributes

Extended attributes in HFS+ provide a mechanism for attaching extra metadata to files and directories. This metadata can include information such as system-specific labels, application settings, and custom icon data. Extended attributes exist alongside the traditional data fork and resource fork found in classic macOS file structures.

Storing these supplemental data components allows specialized applications and the macOS operating system to maintain intricate file properties without cluttering directory listings or file names. By segregating this auxiliary data, HFS+ supports more sophisticated file tagging and advanced application workflows.

What Are the Advantages of HFS+

Listed below are the benefits of HFS+.

Reliability Through Journaling

Journaling increases reliability by recording metadata changes in a discrete area of the disk. This process reduces the risk of data corruption caused by abrupt interruptions or power failures. When the system restarts, HFS+ consults the journal to reconcile incomplete operations, guaranteeing that file directories, allocation tables, and other structural elements remain consistent.

Faster recovery times are realized because the system analyzes a concise log rather than scanning the entire file system for errors. This reliability is particularly beneficial in environments where uptime and data safety are critical.

Efficient B-Tree Implementation

A fundamental design choice in HFS+ is the use of B-tree structures to manage key file system components such as the Catalog File and the Extents Overflow File.

The Catalog File maintains hierarchical relationships between files and folders, storing entries that reference locations, file names, and unique IDs. The Extents Overflow File preserves details about file fragments or extents that cannot fit into the primary catalog entry.

B-tree indexing streamlines operations like searching, inserting, and deleting files by organizing data in a balanced, self-optimizing structure. This strategy lowers retrieval times, accelerates metadata updates, and contributes to overall file system efficiency.

Large Volume Handling

HFS+ addresses volume size constraints encountered in older file systems by supporting storage capacities stretching into the multi-terabyte range. This design permits users to consolidate massive data sets without resorting to the creation of numerous smaller partitions.

Since HFS+ uses 32-bit allocation mappings, it can track significantly more blocks than earlier 16-bit systems. This capability is essential for professional users working with extensive multimedia projects, scientific data analysis, and other high-capacity use cases.

Backward Compatibility

Backward compatibility remains a central advantage of HFS+. Systems still using older macOS versions or classic Mac OS releases remain able to access HFS+ volumes.

This seamless integration prevents data migration complications and ensures users retain access to existing storage media during version transitions. Individuals or organizations that rely on legacy hardware or specialized software environments benefit from a file system that bridges modern functionality with older Mac technologies.

What Are the Disadvantages of HFS+?

Here are the downsides of using HFS+.

Limited Compatibility Outside Apple

HFS+ operates natively within Apple’s operating systems, enabling straightforward read-write capabilities across most macOS releases. However, platforms like Windows and Linux have no built-in, full read-write support for HFS+. Third-party drivers or software utilities are often required to achieve functional cross-platform file sharing.

This limitation complicates data exchange in heterogeneous environments or in scenarios where a single external drive is used across multiple operating systems.

Potential Fragmentation Issues

File fragmentation happens when files are split into non-contiguous blocks on the disk. Fragmentation occurs more frequently when a storage medium is almost full or when frequent file creation, deletion, and modification results in scattered file portions across the volume. Although the B-tree organization helps mitigate some inefficiencies, fragmentation remains a possibility in HFS+.

Fragmentation reduces read-write performance and lengthens seek times, which can become problematic in high-demand workloads. Users may need to employ disk optimization or defragmentation techniques to maintain peak performance over time.

Outdated Design for Modern SSDs

HFS+ was developed when rotational hard drives were the dominant storage technology. Solid-state drives (SSDs) feature distinct characteristics, such as faster access times and the need for wear-leveling, which HFS+ does not directly address at the file system level.

Apple File System (APFS), which supersedes HFS+, incorporates optimizations for SSD performance, including more efficient metadata management, space sharing, and snapshots. HFS+ remains functional on SSDs, but it lacks the sophisticated capabilities that APFS provides for modern solid-state media.

Lack of Native Encryption Features

HFS+ does not include built-in encryption at the file system layer. Encryption features on macOS, like FileVault, operate above the file system, adding an encryption wrapper around the volume rather than integrating cryptographic processes into the core file system structure.

APFS improves upon HFS+ in this regard by offering native encryption that interacts more closely with file system operations. This design difference makes HFS+ less suitable for security-focused environments that prioritize granular, on-disk encryption across all volumes or files.