linux-ext4 - Re: metadata overhead

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Date:   Mon, 3 Sep 2018 17:20:21 -0700
From:   Liu Bo <obuil.liubo@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, "Theodore Y. Ts'o" <tytso@....edu>
Subject: Re: metadata overhead

On Mon, Sep 3, 2018 at 2:53 AM, Jan Kara <jack@...e.cz> wrote:
> Hi,
>
> On Sun 02-09-18 23:58:56, Liu Bo wrote:
>> 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?
>
> Yes, that sounds plausible.
>
>> 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?
>
> So journal takes up some space. Debugfs command:
>
> stat <8>
>
> Inode table takes lots of blocks:
>
> stats
>
> search for "Inode count", multiply by "Inode size". Then there are bitmap
> blocks - count 2 blocks for each group. The rest of overhead should be
> pretty minimal.
>

Thank you so much for the reply, Jan.

Per what you've mentioned, the journal + inode table have taken >14G
in this ext4, so that's a lot of space, good.

And I digged it further, I found that the overhead from (journal +
inode_table + block_bitmap) has been excluded in the output of 'df' as
ext4_statfs() gets buf->f_blocks by

buf->f_blocks = ext4_blocks_count(es) - EXT4_C2B(sbi, overhead);


and ->f_blocks is shown as "Total", but there is still some gap
between "Used" in df (26517428 * 1024) and the summary report of "du
-s" (13347132 * 1024),

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

#du -s /mnt
13347132        /mnt
--------

Now I'm even more curious, any idea where could those gap come from?

thanks,
liubo