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 Figure 12-5- 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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- Create a directory entry that accommodates the new file name.
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- Update the newly created directory entry so that it reflects the original one.
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- Remove the original directory entry.
Without journaling, a failure occurring during the rename operation can leave the file system in any of the following corrupted states:
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- The original directory entry is intact but orphaned LFN entries remain due to a partial directory entry creation.
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- The new directory entry now exists (creation has been completed) but orphaned LFN entries remain due to an uncompleted original directory entry deletion.
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- Two directory entries (both pointing to the same data) now exist: one containing the original name and another one containing the new name.
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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- The original directory entry is intact and everything appears as if nothing had happened.
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- The new directory entry has been created and the original one has been completely deleted, so that the file has been cleanly renamed.