Re: ext4: Used block count in df

[Eric Sandeen <sandeen@redhat.com>]

On 2/11/13 12:36 AM, Adil Mujeeb wrote:
> Thanks Eric.
> 
>>> 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 :(

My only point is, default ext4 statfs behavior is quite complicated, and it
looks like you have found a bug related to the calculation of metadata overhead.

It should only be a reporting issue, and should not cause any runtime issues.

Thanks,
-Eric
 
> Regards,
> Adil