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Date:   Thu, 16 Feb 2023 10:07:39 -0700
From:   Andreas Dilger <>
To:     Jan Kara <>
Cc:     Ojaswin Mujoo <>,
        Ext4 Developers List <>,
        Theodore Ts'o <>,
        Ritesh Harjani <>,,,
        rookxu <>,
        Ritesh Harjani <>
Subject: Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode

On Feb 10, 2023, at 7:37 AM, Jan Kara <> wrote:
> So I belive mballoc tries to align everything (offsets & lengths)
> to powers of two to reduce fragmentation and simplify the work for
> the buddy allocator.  If ac->ac_b_ex.fe_len is a power-of-two, the
> alignment makes sense. But once we had to resort to higher allocator
> passes and just got some random-length extent, the alignment stops
> making sense.

In addition to optimizing for the buddy allocator, the other reason that
the allocations are aligned to power-of-two offsets is to better align
with underlying RAID stripes.  Otherwise, unaligned writes will cause
parity read-modify-write updates to multiple RAID stripes.  This alignment
can also help (though to a lesser degree) with NAND flash erase blocks.

Cheers, Andreas

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