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直接上一张图把，
左边是在ext4文件系统下的dd出来的一个镜像，右边是ext结构体向的表述。
这里是使用dumpe2fs命令得到的信息
caopeng@yep:~/tmp $ dumpe2fs version_da.img
dumpe2fs 1.42 (29-Nov-2011)
Filesystem volume name: version
Last mounted on: <not available>
Filesystem UUID: d8003b02-a900-9650-8909-c6d9835f2dd9
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode filetype extent sparse_super large_file uninit_bg
Filesystem flags: unsigned_directory_hash
Default mount options: (none)
Filesystem state: clean
Errors behavior: Remount read-only
Filesystem OS type: Linux
Inode count: 2016
Block count: 8052
Reserved block count: 0
Free blocks: 6562
Free inodes: 1946
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 7
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 2016
Inode blocks per group: 126
Last mount time: n/a
Last write time: Thu Jan 1 08:00:00 1970
Mount count: 0
Maximum mount count: 10
Last checked: Thu Jan 1 08:00:00 1970
Check interval: 0 (<none>)
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: tea
Journal backup: inode blocks
Journal features: (none)
Journal size: 4096k
Journal length: 1024
Journal sequence: 0x00000001
Journal start: 0
Group 0: (Blocks 0-8051)
Checksum 0x9cd3, unused inodes 0
Primary superblock at 0, Group descriptors at 1-1
Reserved GDT blocks at 2-8
Block bitmap at 9 (+9), Inode bitmap at 10 (+10)
Inode table at 11-136 (+11)
6562 free blocks, 1946 free inodes, 22 directories
Free blocks: 1161, 1164, 1166, 1168, 1378, 1380, 1382, 1384, 1386, 1388, 1390, 1392, 1394, 1397, 1400, 1403, 1406, 1409, 1412, 1415, 1418, 1421, 1424, 1427, 1430, 1433, 1436, 1438, 1440, 1443, 1445, 1447, 1449, 1451, 1453, 1455, 1457, 1459, 1461, 1465, 1469, 1471, 1473, 1495, 1497, 1499, 1516, 1518, 1520, 1522, 1524, 1526, 1531, 1537, 1539, 1541, 1543, 1545, 1547, 1549-8051
Free inodes: 71-2016