Third Extended Filesystem

The third extension to the Ext filesystem, logically known as Ext3, features a journal in which actions performed on the filesystem data are saved. This helps to considerably shorten the run time of fsck following a system crash.21 Since the underlying filesystem concepts not related to the new journal mechanism have remained unchanged in the third version of the filesystem, I will discuss only the new Ext3 capabilities. However, for reasons of space, I will not delve too deeply into their technical implementation.

The transaction concept originates from the database sector, where it helps guarantee data consistency if operations are not completed. The same consistency problem (which is not specific to Ext) arises in

21On filesystems with several hundred gigabytes, consistency checks may take a few hours depending on system speed. This downtime is not acceptable on servers. But even PC users appreciate the fact that consistency checks take just a few seconds rather than several minutes.

filesystems. How can the correctness and consistency of metadata be ensured if filesystem operations are interrupted unintentionally — for example, in the event of a power outage or if a user switches a computer off without shutting it down first?

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