ISO9660
File systems are created to help data structuring, protection, and management on specific storage devices, including optical media. However, the file systems for optical media devices are a lot different from that of hard drives. One of the foremost and pioneering optical media file systems is the ISO9660, which also led to the CDFS format.
ISO9660: Understanding the ISO 9660 File System Standard
The ISO9660 is a standard file system format for optical media storage, and it’s written for different operating systems, ensuring cross-platform compatibility. ISO 9660 follows the path of the High Sierra Format (HSF) tree file system arrangement with dense sequential layouts. Although similar to UNIX and FAT in structure, ISO 9660 is different in its data storage attributes.
Introduction to ISO9660
Introduced in 1988 in a joint development effort from the ISO/IEC and ECMA International, ISO9660 is today a widely recognized file system standard designed for optical media like CDs and DVDs. The intent of its development is to ensure data consistency on optical media devices across different operating systems; hence, it was licensed and sold, resulting in the implementation being written for many operating systems. Since its introduction, ISO9660 has remained a fundamental format for read-only data distribution and archiving on optical discs.
What Is ISO 9660?
ISO 9660 is an international file system standard defined by the International Organization for Standardization (ISO) for optical discs. It establishes a consistent way to store files and directories on read-only media, ensuring accessibility across different computer systems, including Windows, macOS, and Linux. The key objectives of ISO 9660 are:
- Cross-platform compatibility
- Read-only media support to ensure data cannot be further tampered with after writing.
- Hierarchical directory structure to allow organization of files into folders.
- Compatibility with legacy systems (older computer hardware).
Structure of ISO 9660
ISO 9660 employs a hierarchical file system design, similar to UNIX and DOS-based structures. The directory structure consists of a root directory containing subdirectories and files, following strict naming conventions.
Directory Structure and File Naming Conventions:
- 8.3 filename format (Level 1): Filenames can be up to 8 characters long with a 3-character extension (e.g., FILE.TXT).
- Depth limitations: Directories can have a maximum depth of 8 levels.
Uppercase-only filenames: Standard filenames must be in uppercase without spaces.
ISO9660 Levels and Compliance
ISO 9660 defines three levels, each offering different flexibility in file naming and directory structure.
- Level 1:
- Strict 8.3 filename format (max 8 characters, 3-character extension)
- Single data section per file
- Directory depth limited to 8 levels
- Level 2:
- Allows filenames up to 31 characters
- No restriction on the number of file sections
- Better suited for modern applications
- Level 3:
- Supports fragmented files for more efficient storage
- Commonly used in write-once media
Features and Limitations of ISO 9660
Just as ISO 9660 looks beneficial and advantageous, it has its limitations too, and you must know these things. Starting with the features, ISO 9660 has the following:
- Cross-platform readability
- Standardized format
- Ideal for read-only media on CD-ROMs and other optical discs.
Limitations:
- Filename restrictions: Limited character lengths and uppercase-only filenames.
- Lack of modern features: No support for journaling or metadata like in NTFS or ext4.
- Read-only nature: Not designed for rewritable optical media
ISO9660 Extensions
Through extensions, ISO9660 has been improved to be compatible with more systems, OSes, and devices. Some top extensions are:
Rock Ridge: Enforces compatibility with UNIX/Linux systems by allowing long filenames, symbolic links, and POSIX permissions.
Joliet: Developed by Microsoft to support long filenames (up to 64 characters) for Windows systems.
SUSP (System Use Sharing Protocol): The SUSP, IEEE P1281, allows for the inclusion of additional properties in directories. It uses common tags and system-use fields:
- CE: Continuation area
- PD: Padding field
- SP: Sharing protocol indicator
- ST: Sharing protocol terminator
- ER: Extensions reference
- ES: Extension selector
Notwithstanding, no extension allows ISO 9660 to support rewriting. So, the UDF (Universal Disk Format) was introduced as a replacement, supporting rewritable optical media data and meeting modern storage needs.
Applications of ISO9660
Up until today, the ISO9660 format and standard is still widely used for:
- CD-ROMs and DVDs
- Software distribution
- Bootable discs
- Archival purposes
- And more use cases that require optical media data storage
ISO9660 vs. Modern File Systems
Apparently, modern file systems offer more flexibility and advanced features than ISO 9660, but particularly for optical media storage, here’s how ISO 9660 compares to UDF.
| Feature | ISO9660 | UDF |
| Read/write support | Read-only | Read & Write |
| Filename length | 8.3 (Level 1), 31 (Level 2) | Up to 255 characters |
| Directory depth | 8 levels max | No restriction |
| OS compatibility | Universal | Universal, is better for modern systems |
| File Deletion | No | Yes |
While UDF has largely replaced ISO 9660 for modern optical media, ISO 9660 remains relevant for legacy applications and backward compatibility.
Creating ISO9660 File Systems
Creating an ISO9660-compliant file system is commonly done using tools like mkisofs or genisoimage on Linux distros. You need to buy a blank disk, insert it into your system, and runs the commands below.
Commands:
- Using mkisofs:
mkisofs -o mydisk.iso -R -J /path/to/files
- -R: Enables Rock Ridge extensions.
- -J: Enables Joliet extensions.
Using genisoimage:
- genisoimage -o mydisk.iso -iso-level 3 /path/to/files
How to Verify ISO9660 Images
To verify the integrity of an ISO9660 image on a Linux system:
- Check the file system type:
- isoinfo -d -i mydisk.iso
- Mount and inspect contents:
- mount -o loop mydisk.iso /mnt/iso
- ls /mnt/iso
Conclusion
In summary, despite the emergence of more advanced file systems, ISO 9660 remains significant due to its universal compatibility and role in read-only media. It continues to be essential for legacy systems, bootable media, software distributions, and archival storage, due to its simplicity and seamless data access across multiple platforms.