The EXT3 or third extended file system is a journalled file system that is coming into increasing use among users of the Linux operating system. It is the default file system for the Red Hat, Fedora and Debian Linux distributions. Why to migrate from EXT2 to EXT3? Four main reasons: availability, data integrity, speed, and easy transition.
Although its performance and scalability is less attractive than many of its competitors such as ReiserFS and XFS it does have the significant advantage in that it allows in-place upgrades from the popular EXT2 file system without having to backup and restore data.
The EXT3 file system adds, over its predecessor:
H-tree (hashed tree) directory indexes
In-directory file types
Without these, any EXT3 file system is also a valid EXT2 file system. This has allowed well-tested and mature file system maintenance utilities (like fsck) for maintaining and repairing EXT2 file systems to also be used with EXT3 without major changes. It also makes conversion between the two file systems (both forward to EXT3 and backward to EXT2) straightforward.
There are three levels of journalling available in the Linux implementation of EXT3:
Journal, where both metadata and file contents are written to the journal before being committed to the main file system. This improves reliability at a performance penalty because all data has to be written twice.
Writeback, where metadata is journalled but file contents are not. This is faster, but introduces the hazard of out-of-order writes where, for example, files being appended to during a crash may gain a tail of garbage on the next mount.
Ordered, as with writeback, but forces file contents to be written after its associated metadata is. This is thought to be an acceptable compromise between reliability and performance, and hence is the default.
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