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Re: Numbers behind "df" and "tune2fs"

• From: Eric Sandeen <sandeen redhat com>
• To: Nicolas Michel <be nicolas michel gmail com>
• Cc: ext3-users redhat com
• Subject: Re: Numbers behind "df" and "tune2fs"
• Date: Mon, 16 Sep 2013 11:25:37 -0500

```On 9/16/13 9:44 AM, Nicolas Michel wrote:
> Thanks for you help. I also tried adding some other informations as you suggest:
> I can also take into account:
> - "Reserved block count:     XXXXXXX" from tune2fs that gives me the
> number of blocks reserved for root
> - Reserved GDT blocks:      XXX
>
> But I didn't thought about the FS journal. How can I gather
> information about it? (it's size and any other information?)

# dumpe2fs /dev/\$YOUR_DEVICE  | grep Journal
dumpe2fs 1.41.12 (17-May-2010)
Journal inode:            8
Journal backup:           inode blocks
Journal features:         journal_incompat_revoke
Journal size:             128M
Journal length:           32768

But you also need to take into account inode tables, inode
allocation bitmaps, block allocation bitmaps ...

-Eric

> 2013/9/16 Eric Sandeen <sandeen redhat com>:
>> On 9/16/13 5:16 AM, Nicolas Michel wrote:
>>> Hello guys,
>>>
>>> I have some difficulties to understand what really are the numbers
>>> behing "df" and tune2fs. You'll find the output of tune2fs and df
>>> below, on which my maths are based.
>>>
>>> Here are my maths:
>>>
>>> A tune2fs on an ext3 FS tell me the FS size is 3284992 block large. It
>>> also tell me that the size of one block is 4096 (bytes if I'm not
>>> wrong?). So my maths tell me that the disk is 3284992 * 4096 =
>>> 13455327232 bytes or 13455327232 / 1024 /1024 /1024 = 12.53 GB.
>>>
>>> A df --block-size=1 on the same FS tell me the disk is 13243846656
>>> which is 211480576 bytes smaller than what tune2fs tell me.
>>
>> By default, df on extN assumes that metadata used by the filesystem
>> was never available for your use and is not part of the filesystem
>> space.
>>
>> Documentation/filesystems/ext3.txt says:
>>
>> bsddf           (*)     Make 'df' act like BSD.
>> minixdf                 Make 'df' act like Minix.
>>
>> which is pretty unhelpful I suppose.  ;)
>>
>> The mount man page is a little more helpful:
>>
>>        bsddf|minixdf
>>               Set the behaviour for the  statfs  system  call.  The  minixdf
>>               behaviour  is to return in the f_blocks field the total number
>>               of blocks of the filesystem, while the bsddf behaviour  (which
>>               is the default) is to subtract the overhead blocks used by the
>>               ext2 filesystem and not available for file storage.
>>
>> You're seeing the latter behavior.  if you mount with -o minixdf you should
>> see what you expect.  (Too bad there's no "linuxdf?")  :)
>>
>>> In gigabytes, it means:
>>> * for df, the disk is 12.33 GB
>>> * for tune2fs, the disk is 12.53 GB
>>>
>>> I thought that maybe df is only taking into account the real blocks
>>> available for users. So I tried to remove the reserved blocks and the
>>> GDT blocks:
>>> (3284992 - 164249 - 801) * 4096 = 12779282432
>>> or in GB : 12779282432 / 1024 / 1024 / 1024 = 11.90 Gb ...
>>
>> you're on the right track, but you forgot the journal space, all the
>> preallocated inode table blocks, etc.
>>
>> -Eric
>>
>>> My last thought was that "Reserved block" in tune2fs was not only the
>>> reserved blocks for root (which is 5% per default on my system) but
>>> take into account all other reserved blocks fo the fs internal usage.
>>> So:
>>> (3284992 - 164249) * 4096 = 12782563328
>>> In GB : 11.90 Gb (the difference is not significative with a precision of 2.
>>>
>>> So I'm lost ...
>>>
>>> Is someone have an explanation? I would really really be grateful.
>>> Nicolas
>>>
>>> ------------------------------
>>> ---------
>>>
>>> Here is the output of df and tune2fs :
>>>
>>> \$ tune2fs -l /dev/mapper/datavg-datalogslv
>>> tune2fs 1.41.9 (22-Aug-2009)
>>> Filesystem volume name:   <none>
>>> Last mounted on:          <not available>
>>> Filesystem UUID:          4e5bea3e-3e61-4fc8-9676-e5177522911c
>>> Filesystem magic number:  0xEF53
>>> Filesystem revision #:    1 (dynamic)
>>> Filesystem features:      has_journal ext_attr resize_inode dir_index
>>> filetype needs_recovery sparse_super large_file
>>> Filesystem flags:         unsigned_directory_hash
>>> Default mount options:    (none)
>>> Filesystem state:         clean
>>> Errors behavior:          Continue
>>> Filesystem OS type:       Linux
>>> Inode count:              822544
>>> Block count:              3284992
>>> Reserved block count:     164249
>>> Free blocks:              3109325
>>> Free inodes:              822348
>>> First block:              0
>>> Block size:               4096
>>> Fragment size:            4096
>>> Reserved GDT blocks:      801
>>> Blocks per group:         32768
>>> Fragments per group:      32768
>>> Inodes per group:         8144
>>> Inode blocks per group:   509
>>> Filesystem created:       Wed Aug 28 08:30:10 2013
>>> Last mount time:          Wed Sep 11 17:16:56 2013
>>> Last write time:          Thu Sep 12 09:38:02 2013
>>> Mount count:              18
>>> Maximum mount count:      27
>>> Last checked:             Wed Aug 28 08:30:10 2013
>>> Check interval:           15552000 (6 months)
>>> Next check after:         Mon Feb 24 07:30:10 2014
>>> Reserved blocks uid:      0 (user root)
>>> Reserved blocks gid:      0 (group root)
>>> First inode:              11
>>> Inode size:              256
>>> Required extra isize:     28
>>> Desired extra isize:      28
>>> Journal inode:            8
>>> Default directory hash:   half_md4
>>> Directory Hash Seed:      ad2251a9-ac33-4e5e-b933-af49cb4f2bb3
>>> Journal backup:           inode blocks
>>>
>>> \$ df --block-size=1 /dev/mapper/datavg-datalogslv
>>> Filesystem                      1B-blocks      Used   Available Use% Mounted on
>>> /dev/mapper/datavg-datalogslv 13243846656 563843072 12007239680   5% /logs
>>>
>>>
>>
>
>
>

```

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