extents tree

Definition: Data held by storage device such as hard-disk to keep track of where all the non-continguous fragments (i.e. the parts of a file that aren't next to each other) of a large file are being kept.
Misery by extension
The extents tree must be updated every time the file changes in size: thus minor damage to it can make life seriously miserable as it could render a large file unreadable or, and obviously worse, cause a hard-disk crash.

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