最近在研究ext4文件系统，系统同道之人能共同研究、探讨
背景：
在根目录底下
root@f303server:/mnt/sde3# ls -i
14 2.txt 786433 dir1 1048577 dir3 1179649 mkfs
15 3.txt 917505 dir2 11 lost+found 13 sde3.txt
问题：
用黄色标记的几个为目录项，其中lost+found是格式化、挂载文件系统时自带的目录，其余四个是我创建的。那么问题来了，为什么我创建目录结点的inode号那么的大？
解释：
在ext4中，文件系统为了保证数据能更加均匀地分布在磁盘上，所以在创建目录时，尽量均匀地分布在不同的flexible block_group（flexbg）中。且同一个目录下的文件和这个目录的inode号在一个flexbg中：
默认一个flexbg含有16个block group，故 一个flexbg中含有inode结点数为8196 * 16 = 131072 个。所以dir1：786433 位于第6个flexbg中（131072 * 6 + 1 = 786433），在dir1目录下的1.txt，查看其inode结点号为 786434。
同理可以计算其他的三个目录分别在第7、8、9个flexbg中
ps:
1怎么查看一个group有多少个inode结点：
debugfs /dev/sde3
stats
显示如下
debugfs
Stats
Filesystem volume name: <none>
Last mounted on: /mnt/sde3
Filesystem UUID: fea02b9a-41d1-4bf1-96ce-d3298d67778f
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: user_xattr acl
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 1310720
Block count: 5242880
Reserved block count: 262144
Free blocks: 5116587
Free inodes: 1310705
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 1022
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512 #Inode table所占的块数目
Flex block group size: 16 #下面前16个inode bitmap所在
Filesystem created: Mon Jul 28 08:40:58 2014
Last mount time: Tue Jul 29 19:53:03 2014
Last write time: Tue Jul 29 19:53:03 2014
Mount count: 8
Maximum mount count: -1
Last checked: Mon Jul 28 08:40:58 2014
Check interval: 0 (<none>)
Lifetime writes: 454 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: 0da457a4-59ff-402a-9181-c66156ea5991
Journal backup: inode blocks
Directories: 6
Group 0: block bitmap at 1025, inode bitmap at 1041, inode table at 1057
23513 free blocks, 8178 free inodes, 2 used directories, 8176 unused inodes
[Checksum 0x7a6d]
Group 1: block bitmap at 1026, inode bitmap at 1042, inode table at 1569
31740 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Checksum 0x2c05]
Group 2: block bitmap at 1027, inode bitmap at 1043, inode table at 2081
32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Block not init, Checksum 0x7c29]
Group 3: block bitmap at 1028, inode bitmap at 1044, inode table at 2593
31743 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Checksum 0x1e2b]延迟初始化
………..
……..
Group 12: block bitmap at 1037, inode bitmap at 1053, inode table at 7201
32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Block not init, Checksum 0x30ab]
Group 13: block bitmap at 1038, inode bitmap at 1054, inode table at 7713
32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Block not init, Checksum 0x8f81]
Group 14: block bitmap at 1039, inode bitmap at 1055, inode table at 8225
32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Block not init, Checksum 0x8aa2]
Group 15: block bitmap at 1040, inode bitmap at 1056, inode table at 8737
32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Block not init, Checksum 0x350d]
Group 16: block bitmap at 524288, inode bitmap at 524304, inode table at 524320
24544 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
[Inode not init, Checksum 0xb89c]
2.怎么查看flexbg包含16个block group
从上面的debugfs的group 显示结果可以看出，group 0至group 15的block bitmap，inode bitmap、inode table是连续存放的。而从group 16开始元数据不再连续了，这也说明flexbg为16个block group。
