linux-ext4 - Re: Repeatable block allocation problem.

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Message-ID: <4c8249e3-8429-2d91-0431-6bc3a319fae5@redhat.com>
Date:   Mon, 17 Sep 2018 10:44:25 -0500
From:   Eric Sandeen <esandeen@...hat.com>
To:     Joshi <joshiiitr@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: Repeatable block allocation problem.

On 9/17/18 6:22 AM, Joshi wrote:
> 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.

Userspace cannot control that.

> 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.

Userspace doesn't get to pick physical locations for allocations, you cannot
write a test which depends on doing so.

-Eric