From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <542D3378-3214-4D0D-AA63-5A149E2B00EE@dilger.ca>
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Subject: Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode
 i_prealloc_list
Date:   Thu, 16 Feb 2023 10:07:39 -0700
In-Reply-To: <20230210143753.ofh6wouk7vi7ygcl@quack3>
Cc:     Ojaswin Mujoo <ojaswin@linux.ibm.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Ritesh Harjani <riteshh@linux.ibm.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        rookxu <brookxu.cn@gmail.com>,
        Ritesh Harjani <ritesh.list@gmail.com>
To:     Jan Kara <jack@suse.cz>
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On Feb 10, 2023, at 7:37 AM, Jan Kara <jack@suse.cz> 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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