linux-ext4 - Re: ext4: Used block count in df

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Message-ID: <5113DB2D.4000305@redhat.com>
Date:	Thu, 07 Feb 2013 10:49:49 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Adil Mujeeb <mujeeb.adil@...il.com>
CC:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: ext4: Used block count in df

On 2/7/13 12:39 AM, Adil Mujeeb wrote:
> Hi,
> 
> I have an observation on EXT4 filesystem. I created filesystem of size
> 1TB, 4TB, and 7TB and then checked the output of df command.

Telling us which version of e2fsprogs and which kernel would be helpful,
but:

> df command showed the number of 1KB blocks used. The result was:
> 1TB: 204056
> 4TB: 198680
> 7TB: 181784

extN makes df complicated in several ways.

It reserves blocks for the superuser (5% by default) and also uses a lot
of blocks up-front for filesytem metadata - inode tables, block bitmaps,
and the like.

But what you are seeing here is this:

It also defaults to "bsd df" which does not count filesystem
metadata when telling you about the number of blocks used.  So in theory,
a freshly made fs should actually tell you 0 blocks used, I think.

Looking at the dumpe2fs output for the 4t file, I see:

# dumpe2fs -h 4tfile-ext4 | grep -i block
dumpe2fs 1.41.12 (17-May-2010)
Block count:              1073741824
Reserved block count:     53687091
Free blocks:              1056843748
...

and 1073741824-1056843748 is 16898076 4k blocks, or 67592304 1k blocks
actually used.

If we ask for "minix df" by mounting with -o minixdf which is true blocks used, we get:

# df 4t-ext4/
Filesystem           1K-blocks      Used Available Use% Mounted on
/mnt/test2/mkfs-test/4tfile-ext4
                     4294967296  67592304 4012626628   2% /mnt/test2/mkfs-test/4t-ext4

I'd say this appears to be a slight inaccuracy in ext4_statfs, coupled with
the strangeness of the "bsd df" reporting.  It is apparently miscalculating
the filesystem metadata "overhead."

> I performed the same on XFS and the result was:
> 1TB: 32928
> 4TB: 32928
> 7TB: 33024

XFS is straightforward; blocks used for metadata count as "used."
Every other block is free and available.
No fiddling around, just like with the minixdf mount option for extN.

-Eric

> EXT4 result shows with increasing filesystem size, the number of used
> blocks decreased. I dont have idea about low level implementation but
> I am curious why it is so?
> 
> Thanks.
> 
> Regards,
> Adil
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