ext4, the successor of ext3, is a Linux journaling file system and a cornerstone of storage management since its introduction in 2008. It supports large file systems and many files, making it a popular choice for everything from personal computing to enterprise-level data centers.
What Is ext?
The "ext" file system, short for "extended file system," was the first specifically designed for the Linux kernel. Introduced in 1992, it was created to overcome the limitations of the Minix filesystem, which was used in early versions of Linux. The ext file system brought several improvements, such as the support for file system metadata, which allowed for more efficient management of files and directories. It supported larger file sizes and a greater number of files than Minix, which was crucial for Linux's adoption and growth.
However, the ext file system had its limitations, such as a lack of a journaling feature, which led to the development of its successors:
- ext2, introduced in 1993, was non-journaling but offered better performance and reliability than the original ext.
- ext3, which came out in 1999, added journaling capabilities to improve data integrity on system crashes.
- ext4, released in 2008, further expanded the capabilities in terms of size and performance.
Each iteration was designed to address the evolving needs of Linux systems while maintaining compatibility with previous versions.
What Is ext4?
The ext4, or fourth extended file system, is a journaling file system for Linux, developed as the successor to ext3. It introduces several improvements over its predecessor, enhancing performance, reliability, and storage capacity.
ext4 supports larger file systems and handles more files than ext3, making it suitable for modern storage needs. It includes features like delayed allocation, which improves performance and reduces fragmentation by waiting to decide the location of the file system blocks until they are actually written.
The ext4 file system also implements a 48-bit block addressing scheme, enabling it to support disk sizes up to 1 exabyte and individual files up to 16 terabytes in size. This file system is designed to extend storage limits and increase access speed without compromising data integrity, maintaining ext3's ease of upgrade while allowing for backward compatibility.
ext4 Features
Here are the key characteristics and features of ext4:
- Large file system support. ext4 can support volumes with sizes up to 1 exbibyte (EiB) and files with sizes up to 16 tebibytes (TiB), which is a significant increase over its predecessor.
- Extents. Unlike ext3, which uses a bitmap (block mapping) system, ext4 uses extents, which are a more efficient method of file block allocation that improves performance with large files and reduces fragmentation.
- Backward compatibility. ext4 is backward compatible with ext3 and ext2, meaning that it can mount file systems created with these earlier versions.
- Journaling. ext4 retains the use of a journaling mechanism, which helps protect the integrity of the file system against sudden shutdowns, hardware crashes, and other types of system failures.
- Delayed allocation. This feature improves performance and reduces fragmentation by delaying the allocation of blocks until they are actually needed for data storage.
- Increased subdirectory limit. ext4 supports an unlimited number of subdirectories, unlike ext3, which is limited to 32,000.
- Timestamps. ext4 supports timestamps with nanosecond resolution, which is an improvement over the second resolution used in ext3. It also includes support for dates up to the year 2446 through the use of 48-bit date fields.
- Online defragmentation. ext4 supports online defragmentation, allowing for the defragmentation of the file system without unmounting it, which is crucial for systems that require continuous uptime.
- Checksums for journaling. Ext4 can use checksums in the journal to improve reliability and crash recovery speed.
Advantages and Disadvantages of ext4
ext4 offers a variety of advantages as well as some downsides users should be aware of.
Advantages
Here are some key advantages of the ext4 file system that make ext4 a robust, efficient, and versatile choice for various Linux distributions, contributing to its widespread adoption across different sectors and use cases:
- Increased storage limits. ext4 can handle larger volumes and files significantly better than its predecessors. It supports volumes up to 1 exabyte and files up to 16 terabytes in size. This capacity makes it suitable for enterprise-level applications and data-intensive tasks, accommodating the growing demands for storage.
- Improved performance. ext4 introduces several features that enhance its performance. It uses extents, which are contiguous blocks of storage, reducing fragmentation and improving the speed of file access and storage. Delayed allocation improves I/O performance by optimizing the way data is written to disk, thereby enhancing the overall efficiency of the file system.
- Journaling. One of the main strengths of ext4 is its journaling capability, which helps protect the integrity of data by maintaining a log of ongoing transactions. In the event of a system crash or power failure, this feature allows the file system to recover more quickly and cleanly by replaying the journal.
- Backward compatibility. ext4 is backward compatible with ext2 and ext3 file systems. This feature allows users to mount older file systems as ext4 without needing to convert all data, which provides great flexibility and ease of upgrade.
- Online defragmentation. ext4 supports online defragmentation, enabling users to defragment the file system while it's still mounted and in use. This feature is beneficial in environments where downtime must be minimized.
- Persistent preallocation. ext4 allows for the preallocation of disk space to files before they are written. This feature is useful for applications that need to guarantee they have enough space for data before starting an operation, such as multimedia recording or scientific data processing.
Disadvantages
Here's a list of the disadvantages of the ext4 file system, with explanations for each:
- Lack of built-in snapshots and checksums. Unlike some modern file systems like Btrfs or ZFS, ext4 does not natively support snapshots or checksums for files and metadata. This means it can't automatically capture and manage versions of the file system or verify the integrity of data at a block level, which is crucial for preventing data corruption.
- Limited scalability. While ext4 supports very large file sizes and file system capacities, it lacks some of the more advanced scalability features of newer file systems like dynamic inode allocation (found in Btrfs), which adjusts automatically as storage needs grow, and real-time disk optimization features.
- Complex recovery. Due to its journaling nature, ext4 can be more complex to recover in cases of severe corruption or failures compared to simpler, non-journaling file systems like ext2. While the journaling feature enhances data integrity after unexpected shutdowns, it can complicate recovery efforts because the journal must be replayed or cleared correctly.
- Write amplification. ext4 may cause write amplification, especially on SSDs, where each write operation involves multiple writes due to journaling. This can reduce the lifespan of SSDs over time as they have a limited number of write cycles.
- Data aging. ext4 is not optimized for extremely long-term data storage where the file system is not frequently accessed but still needs to remain reliable over time. The journaling system can lead to scenarios where older data isn't refreshed or verified as it would be in file systems designed for archival purposes.
- Performance overhead. The journaling feature, while beneficial for data integrity, introduces a performance overhead. Every write operation has to be recorded in the journal, which can slow down write speeds, especially in environments with high I/O requirements.
- No native encryption. ext4 does not provide native encryption support within the file system itself. Users must rely on third-party tools or kernel-level encryption to secure data at rest, which can complicate setups and potentially impact performance.
ext4 vs. ext3
ext4 and ext3 are both journaling file systems used primarily in Linux environments, but ext4 introduces several enhancements over ext3, making it more suitable for modern computing needs.
ext4 supports much larger file systems and files; it can handle volumes up to 1 exabyte and file sizes up to 16 terabytes, compared to the 16-terabyte file system limit and 2-terabyte file size limit in ext3. ext4 also incorporates extent-based file storage, replacing the traditional block mapping approach used in ext3, significantly improving performance with large files and reducing fragmentation. Additionally, ext4 features delayed allocation and offers faster file system checking compared to ext3.
Moreover, ext4 includes new capabilities that are not present in ext3, such as support for unlimited subdirectories (ext3 supports only up to 32,000 subdirectories), online defragmentation, and a more robust journaling system that can optionally operate in a "writeback" mode. This mode allows for more flexibility in how data is written to the disk, potentially increasing throughput.
ext4 also maintains backward compatibility with ext3, meaning it is possible to mount ext3 file systems as ext4 without modifying the disk data. This ease of upgrade, coupled with improved performance and capacity, makes ext4 an appealing choice for new installations and upgrades from ext3, although ext3 is still in use due to its stability and the extensive testing it has undergone over the years.
