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

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Subject: Any fast way to make an ever pre-allocated extent become unwritten again?
From: Joe Hsu <nagual.hsu@xxxxxxxxx>
Date: Sun, 24 May 2009 15:27:02 +0800
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    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).
-- 
The sun is shinny but the ice is slippery.

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