In order to understand how the journaling module works, you should first understand how API-level operations relate to the underlying FAT layer operations. As seen in the figure below Figure - Relation between API and FAT layer operations, an API level operation is made of one or more top-level FAT operations which, in turn, are made of one or more low-level FAT operations.
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Take a file rename operation, for example. The API-level rename operation involves one top-level FAT rename operation and the following low-level FAT sub-operations:
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Using the journaling module, any of the previous corrupted states would be either rolled back or completed upon volume remounting. This is made possible because, prior to performing any low-level FAT operation, the journaling system logs recovery information in a special file called the journal file. By reverting or completing successive underlying low-level FAT operations, the journaling module also allows top-level FAT operations to be reverted or completed, thus making them atomic (see section 12-6 “Optional Journaling System” Optional Journaling System). In our previous example, the journaled rename operation could only have on of the two following outcomes:
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