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Message-ID: <CA+1E3rLzX0Urd+s9ro1Q+wUoht6uFD1TOt5thGQh+ZseiamKcA@mail.gmail.com>
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
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