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

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Date:	Wed, 13 Feb 2013 10:46:43 +0530
From:	Adil Mujeeb <mujeeb.adil@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: ext4: Used block count in df

Hi Eric,

On Tue, Feb 12, 2013 at 9:31 PM, Eric Sandeen <sandeen@...hat.com> wrote:
> On 2/12/13 12:14 AM, Adil Mujeeb wrote:
>> Hi,
>>
>>> 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.
>>
>> I see.
>> Where should I report this issue to get it confirm by developers?
>
> Here is fine.  :)
>
> It would be good to file a bug on bugzilla.kernel.org too if you like.
>
> The problem is, I think ext4's metadata behavior has gotten so complex,
> the consensus so far seems to be to just accept the inaccuracy in this
> style of df reporting:
>
>  * Note: calculating the overhead so we can be compatible with
>  * historical BSD practice is quite difficult in the face of
>  * clusters/bigalloc.  This is because multiple metadata blocks from
>  * different block group can end up in the same allocation cluster.
>  * Calculating the exact overhead in the face of clustered allocation
>  * requires either O(all block bitmaps) in memory or O(number of block
>  * groups**2) in time.  We will still calculate the superblock for
>  * older file systems --- and if we come across with a bigalloc file
>  * system with zero in s_overhead_clusters the estimate will be close to
>  * correct ...
>
> but it is odd behavior, and filing a bug would probably be good.

I filed the bug in bugzilla. https://bugzilla.kernel.org/show_bug.cgi?id=53741
Thank for all your inputs :)

Regards,
Adil

>
> -Eric
>
>>> It should only be a reporting issue, and should not cause any runtime issues.
>>
>> OK, I understand.
>>
>> Thanks,
>> Adil
>>
>> On Mon, Feb 11, 2013 at 11:02 PM, Eric Sandeen <sandeen@...hat.com> wrote:
>>> 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
>>>
>>>
>
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