Thanks Eric.

quoted

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:

its 1.41.12.

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.

I also thinks so. But with this assumption, the number of 1KB blocks
used should increase as per filesystem size increase. No?

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.

Agree if "bsd df" assumes so.

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."

In your example, dumpe2fs and minix df both are reporting same value, isn't it?

I am still not able to understand why increasing the filesystem size
decreases used 1K block count :(
Am I missing some basic things here? Sorry if i am not able to catch
your point :(

Regards,
Adil

On Thu, Feb 7, 2013 at 10:19 PM, Eric Sandeen [off-list ref] wrote:

On 2/7/13 12:39 AM, Adil Mujeeb wrote:

quoted

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:

quoted

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."

quoted

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

quoted

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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