Any fast way to make an ever pre-allocated extent become unwritten again?

[Joe Hsu]

    I do really need this. When pre-allocated, files are not
fragmented. After writing data to these files, I just want to make the
extent un-written again, using the same allocated blocks. I can only
initialize these files as un-fragmented in the beginning and I don't
want to over-write them simply after they are fully written because
over-writings can cause input traffic(partial updating of some
blocks), thus lowering output performance.

    Thanks if any one can give me some idea. (Except dirty hacks into
xfs filesystem kernel codes).
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