linux-ext4 - Re: ext4 offers less blocks than other fs

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Message-ID: <CAGBYx2Yr6yGmQ8c=34CCW=v0g5-MBFhN3ztyd_p+qEBer1NyCg@mail.gmail.com>
Date:	Wed, 16 Nov 2011 09:43:33 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: ext4 offers less blocks than other fs

On Wed, Nov 16, 2011 at 3:51 AM, Jan Engelhardt <jengelh@...ozas.de> wrote:
> Hi.
>
>
> In moving away from some ntfs-3g filesystem to ext4 (migration of a
> Windows-esque fileserver), I see a rather steep metadata use with ext4
> such that moving the data was not completely possible. Per df:
>
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/dm-5            1953514324 1952543856    970468 100% /mnt/old
> /dev/mapper/data     1922858888 1922858880         0 100% /mnt/new
> [attempted a rsync as root]
>
> The blockdev size is just minimally different:
> 20:32 nas:~ # blockdev --getsize64 /dev/dm-5
> 2000398671872
> 20:32 nas:~ # blockdev --getsize64 /dev/dm-6
> 2000396836864
>
> And there is a 29 GB(!) difference between 1953514324 and 1922858888,
> so my question is: where is all of that going?
>
> (e2fsprogs 1.41.14)
> Filesystem volume name:   data
> Last mounted on:          /mnt/new
> Filesystem UUID:          807fec49-7a8a-3520-a648-be677475377c
> Filesystem magic number:  0xEF53
> Filesystem revision #:    1 (dynamic)
> Filesystem features:      has_journal ext_attr resize_inode dir_index filetype
> needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg
> dir_nlink extra_isize
> Filesystem flags:         signed_directory_hash
> Default mount options:    (none)
> Filesystem state:         clean
> Errors behavior:          Continue
> Filesystem OS type:       Linux
> Inode count:              122101760
> Block count:              488378134
> Reserved block count:     24418906
> Free blocks:              480664715
> Free inodes:              122101749
> First block:              0
> Block size:               4096
> Fragment size:            4096
> Reserved GDT blocks:      907
> Blocks per group:         32768
> Fragments per group:      32768
> Inodes per group:         8192
> Inode blocks per group:   512
> Flex block group size:    16
> Filesystem created:       Mon Nov 14 20:43:23 2011
> Last mount time:          Mon Nov 14 20:43:55 2011
> Last write time:          Mon Nov 14 20:43:55 2011
> Mount count:              1
> Maximum mount count:      26
> Last checked:             Mon Nov 14 20:43:23 2011
> Check interval:           15552000 (6 months)
> Next check after:         Sat May 12 21:43:23 2012
> Lifetime writes:          144 MB
> 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:      5e2619e2-7683-4589-b390-57e569444705
> Journal backup:           inode blocks
> Journal features:         (none)
> Journal size:             128M
> Journal length:           32768
> Journal sequence:         0x00002751
> Journal start:            4341
> [Details about groups 0-14904 omitted.]
Hi,

The system metadata can be calculated.

The inode blocks per group is 512, besides there are 1 block bitmap
block and 1 inode bitmap block per group.  The above filesystem has
488378134 blocks, thus there are 14905 block groups.  So the group
tables (inode table, inode bitmap, block bitmap) take up  7661170
blocks. In addition, journal uses 32768 blocks and group descriptor
uses some blocks.

Yongqiang.
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-- 
Best Wishes
Yongqiang Yang
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