linux-ext4 - Re: Repeatable block allocation problem.

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Date:   Mon, 17 Sep 2018 11:00:59 -0500
From:   Eric Sandeen <sandeen@...hat.com>
To:     Joshi <joshiiitr@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: Repeatable block allocation problem.

On 9/17/18 10:59 AM, Joshi wrote:
> Sorry if i gave the impression that i needed something in user-mode only. I am fine even if change involves modifying ext4 source.

Perhaps you can describe what defrag behavior you are trying to test,
and we can offer some suggestions for good ways to construct that test?

There are already a handful of defrag tests in xfstests, fwiw.

Thanks,
-Eric

> On Mon, 17 Sep 2018, 21:14 Eric Sandeen, <esandeen@...hat.com <mailto:esandeen@...hat.com>> wrote:
> 
>     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
>