linux-ext4 - metadata overhead

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Message-ID: <CANQeFDABkvrjP+bRFJzEEPBasWrsvMPaqxwuSrxK1kDBqpJs7A@mail.gmail.com>
Date:   Sun, 2 Sep 2018 23:58:56 -0700
From:   Liu Bo <obuil.liubo@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>
Subject: metadata overhead

Hi,

My question is,
Is there a way to calculate how much space metadata has occupied?

So the case I've run into is that 'df /mnt' shows

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sdc              882019232  26517428 811356348   4% /mnt

but 'du -s /mnt' shows
13347132        /mnt

And this is a freshly mounted ext4, so no deleted files or dirty data exist.

The kernel is quite old (2.6.32), but I was just wondering, could it
be due to metadata using about 13G given the whole filesystem is 842G?

I think it has nothing to do with "Reserved block counts" as df
calculates "Used" in ext4_statfs() by "buf->f_blocks - buf->f_bfree".

So if there is a way to know the usage of metadata space, via either
manual analysis from the output of dumpe2fs/debugfs or a tool, could
you please suggest?

thanks,
liubo