[formerly_proven]

What this does on typical extent-based file systems is split the extent of the file at the given location (which means these operations can only be done with cluster granularity) and then insert a third extent. i.e. calling INSERT_RANGE once will give you a file with at least three extents (fragments). This, plus the mkfs-options-dependent alignment requirements, makes it really quite uninteresting for broad use in a similar fashion as O_DIRECT is uninteresting.

[cubefox]

Well, better an uninteresting solution than a solution which is actively terrible: appending changes to a PDF, which will inflate its size and cause data leakage.