linux-ext4 - Defrag and delayed-allocation on Ext4

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Date:   Wed, 29 Aug 2018 21:52:32 +0530
From:   Joshi <joshiiitr@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: Defrag and delayed-allocation on Ext4

Possibly a newbie question for the list, excuse my ignorance.
I saw e4defrag code, which fallocates a temporary file in the outset.

I imagine that extents would not get allocated to the this file at
this time due to delayed-allocation feature (extent/block allocations
should happen post writes which seem to happen much later, during
extents copying between target-file and temporary-file).

However, e4defrag needs extent information just after fallocate() to
determine whether it is wise to go-ahead with defrag process i.e.
whether new file is any less-fragmented than target one. I wonder how
this decision-making is possible if fallocate does not allocate
extents/blocks.

I'd appreciate the thoughts.

-- 
Joshi