linux-ext4 - Repeatable block allocation problem.

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Date:   Mon, 17 Sep 2018 16:52:00 +0530
From:   Joshi <joshiiitr@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: Repeatable block allocation problem.

I've been doing some defrag related tests, and for that I needed to be
able to create file with same set of block numbers (i.e. extents), for
at least two times.
May I know if there is any randomness in Ext4 allocator, and if there
is any, can I disable it for the purpose of getting repeatable
block-allocation patterns.

Here are experiment details -
For a 100K file (created using dd if=/dev/zero of=/mnt/f100k bs=4K
count=100, oflag=direct) I got extent info in one run as this -

File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..      15:      34816..     34831:     16:
   1:       16..      99:      33824..     33907:     84:      34832: last,eof

while in the the second run I got somewhat different runs -
File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:      34816..     34816:      1:
   1:        1..      15:      34320..     34334:     15:      34817:
   2:       16..      99:      33824..     33907:     84:      34335: last,eof

Each run beings with a mkfs.ext4 with lazy inode/journal
initialization disabled.

Thanks,
-- 
Joshi