解析ext4文件系统的布局

对于ext2文件系统的布局可能如下图所示

显然，图片中展示的是ext2文件系统的第一个块组（因为超级快位于起始1024字节后），可以看出对于一个block group来说，Super Block和Group Descriptors（一般1block）以及Reserverd GDT Blocks后面跟的Block Bitmap为1个块大小，同样inode Bitmap也是一个块大小,inode Table占固定块大小，也就是说从硬盘数据的布局来看，一个标准块组里面只包含自身块组的的block bitmap、inode Bitmap以及inode Table，可能包括super block和group description table的备份。一个典型的例子如下图所示：

但是，我在使用dumpe2fs命令查看ubuntu的ext4文件系统时，发现从ext2块组的硬盘布局来说，一个块组所包括的块范围中可能并不止是包含自己块组的block bitmap、inode Bitmap以及inode Table，一个块组内可能存放着其他块组的block bitmap、inode Bitmap以及inode Table。它是每16个块组为单位，将这16个块组的所有block bitmap、inode Bitmap以及inode Table都放在16个块组的第一个块组内，同理，某个快组范围内的块可能不包括自己块组的block bitmap,inode bitmap和inode Table.具体见下面ext4文件系统结构信息。

Filesystem volume name:   <none>Last mounted on:          /Filesystem UUID:          0fbdfdd4-80ea-4962-84e8-9173c5ef1cb5Filesystem magic number:  0xEF53Filesystem 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_isizeFilesystem flags:         signed_directory_hash Default mount options:    user_xattr aclFilesystem state:         cleanErrors behavior:          ContinueFilesystem OS type:       LinuxInode count:              6037504Block count:              24146432Reserved block count:     1207321Free blocks:              20876199Free inodes:              5787665First block:              0Block size:               4096Fragment size:            4096Reserved GDT blocks:      1018Blocks per group:         32768Fragments per group:      32768Inodes per group:         8192Inode blocks per group:   512Flex block group size:    16Filesystem created:       Sun Jan  8 01:10:30 2017Last mount time:          Sat Feb 18 11:26:16 2017Last write time:          Sat Feb 18 19:26:08 2017Mount count:              150Maximum mount count:      -1Last checked:             Sun Jan  8 01:10:30 2017Check interval:           0 (<none>)Lifetime writes:          31 GBReserved blocks uid:      0 (user root)Reserved blocks gid:      0 (group root)First inode:              11Inode size:          256Required extra isize:     28Desired extra isize:      28Journal inode:            8First orphan inode:       2250909Default directory hash:   half_md4Directory Hash Seed:      4c440967-7d09-4b9b-b3be-74a8d8bc47caJournal backup:           inode blocksJournal features:         journal_incompat_revokeJournal size:             128MJournal length:           32768Journal sequence:         0x0001f82eJournal start:            1Group 0: (Blocks 0-32767) [ITABLE_ZEROED]  Checksum 0x5119, unused inodes 5265  Primary superblock at 0, Group descriptors at 1-6  Reserved GDT blocks at 7-1024  Block bitmap at 1025 (+1025), Inode bitmap at 1041 (+1041)  Inode table at 1057-1568 (+1057)  22919 free blocks, 5287 free inodes, 592 directories, 5265 unused inodes  Free blocks: 9255-9350, 9354-9500, 9529-10104, 10461-10462, 10490-10492, 10673-32767  Free inodes: 1219-1226, 1335-1345, 1637-1639, 2928-8192Group 1: (Blocks 32768-65535) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xd779, unused inodes 8192  Backup superblock at 32768, Group descriptors at 32769-32774  Reserved GDT blocks at 32775-33792  Block bitmap at 1026 (bg #0 + 1026), Inode bitmap at 1042 (bg #0 + 1042)  Inode table at 1569-2080 (bg #0 + 1569)  23155 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 34151-34160, 34784-34812, 34868-34904, 34906-35013, 35064-35071, 35182-35839, 35865-36735, 36806-38911, 38936-38938, 39054-39057, 39061-39080, 39082-39849, 39932-39934, 39936-39939, 39948-39976, 39980-40027, 41007-41009, 41015-41016, 41018-41020, 41022, 41651-41653, 41655, 41694-41696, 41761, 41767-41830, 41832-41906, 41922, 41934, 42028-42029, 42033-55295, 56576-56622, 56640-56656, 56832-56879, 57344-59391, 59538-60415, 60896-60912, 63565-65535  Free inodes: 8193-16384Group 2: (Blocks 65536-98303) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x2ad8, unused inodes 8192  Block bitmap at 1027 (bg #0 + 1027), Inode bitmap at 1043 (bg #0 + 1043)  Inode table at 2081-2592 (bg #0 + 2081)  28124 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 65536-71423, 71458-71551, 71627-71679, 73728-74541, 74981-75775, 77824-98303  Free inodes: 16385-24576Group 3: (Blocks 98304-131071) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x7ac4, unused inodes 8192  Backup superblock at 98304, Group descriptors at 98305-98310  Reserved GDT blocks at 98311-99328  Block bitmap at 1028 (bg #0 + 1028), Inode bitmap at 1044 (bg #0 + 1044)  Inode table at 2593-3104 (bg #0 + 2593)  29970 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 99355-99359, 99391-101375, 102400-102911, 102964-103423, 103988-116223, 116300-131071  Free inodes: 24577-32768Group 4: (Blocks 131072-163839) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xafa8, unused inodes 8192  Block bitmap at 1029 (bg #0 + 1029), Inode bitmap at 1045 (bg #0 + 1045)  Inode table at 3105-3616 (bg #0 + 3105)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 131072-163839  Free inodes: 32769-40960Group 5: (Blocks 163840-196607) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x35b1, unused inodes 8192  Backup superblock at 163840, Group descriptors at 163841-163846  Reserved GDT blocks at 163847-164864  Block bitmap at 1030 (bg #0 + 1030), Inode bitmap at 1046 (bg #0 + 1046)  Inode table at 3617-4128 (bg #0 + 3617)  31743 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 164865-196607  Free inodes: 40961-49152Group 6: (Blocks 196608-229375) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x7786, unused inodes 8192  Block bitmap at 1031 (bg #0 + 1031), Inode bitmap at 1047 (bg #0 + 1047)  Inode table at 4129-4640 (bg #0 + 4129)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 196608-229375  Free inodes: 49153-57344Group 7: (Blocks 229376-262143) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xadb9, unused inodes 8192  Backup superblock at 229376, Group descriptors at 229377-229382  Reserved GDT blocks at 229383-230400  Block bitmap at 1032 (bg #0 + 1032), Inode bitmap at 1048 (bg #0 + 1048)  Inode table at 4641-5152 (bg #0 + 4641)  31743 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 230401-262143  Free inodes: 57345-65536Group 8: (Blocks 262144-294911) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xd0b4, unused inodes 8192  Block bitmap at 1033 (bg #0 + 1033), Inode bitmap at 1049 (bg #0 + 1049)  Inode table at 5153-5664 (bg #0 + 5153)  32758 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 262144-267903, 267913-267974, 267976-294911  Free inodes: 65537-73728Group 9: (Blocks 294912-327679) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x300f, unused inodes 8192  Backup superblock at 294912, Group descriptors at 294913-294918  Reserved GDT blocks at 294919-295936  Block bitmap at 1034 (bg #0 + 1034), Inode bitmap at 1050 (bg #0 + 1050)  Inode table at 5665-6176 (bg #0 + 5665)  31743 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 295937-327679  Free inodes: 73729-81920Group 10: (Blocks 327680-360447) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xe32a, unused inodes 8192  Block bitmap at 1035 (bg #0 + 1035), Inode bitmap at 1051 (bg #0 + 1051)  Inode table at 6177-6688 (bg #0 + 6177)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 327680-360447  Free inodes: 81921-90112Group 11: (Blocks 360448-393215) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x63e2, unused inodes 8192  Block bitmap at 1036 (bg #0 + 1036), Inode bitmap at 1052 (bg #0 + 1052)  Inode table at 6689-7200 (bg #0 + 6689)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 360448-393215  Free inodes: 90113-98304Group 12: (Blocks 393216-425983) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xa97c, unused inodes 8192  Block bitmap at 1037 (bg #0 + 1037), Inode bitmap at 1053 (bg #0 + 1053)  Inode table at 7201-7712 (bg #0 + 7201)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 393216-425983  Free inodes: 98305-106496Group 13: (Blocks 425984-458751) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x1656, unused inodes 8192  Block bitmap at 1038 (bg #0 + 1038), Inode bitmap at 1054 (bg #0 + 1054)  Inode table at 7713-8224 (bg #0 + 7713)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 425984-458751  Free inodes: 106497-114688Group 14: (Blocks 458752-491519) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x1375, unused inodes 8192  Block bitmap at 1039 (bg #0 + 1039), Inode bitmap at 1055 (bg #0 + 1055)  Inode table at 8225-8736 (bg #0 + 8225)  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 458752-491519  Free inodes: 114689-122880Group 15: (Blocks 491520-524287) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xc64f, unused inodes 8192  Block bitmap at 1040 (bg #0 + 1040), Inode bitmap at 1056 (bg #0 + 1056)  Inode table at 8737-9248 (bg #0 + 8737)  27511 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 491520-504959, 505048-516095, 521237-522911, 522940-524287  Free inodes: 122881-131072Group 16: (Blocks 524288-557055) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0xe6b2, unused inodes 8192  Block bitmap at 524288 (+0), Inode bitmap at 524304 (+16)  Inode table at 524320-524831 (+32)  24544 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 532512-557055  Free inodes: 131073-139264Group 17: (Blocks 557056-589823) [INODE_UNINIT, ITABLE_ZEROED]  Checksum 0x6b5f, unused inodes 8192  Block bitmap at 524289 (bg #16 + 1), Inode bitmap at 524305 (bg #16 + 17)  Inode table at 524832-525343 (bg #16 + 544)  28480 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes  Free blocks: 561244-561279, 561380-589823  Free inodes: 139265-147456

参考资料：

ext2文件系统

ext4文件系统维基百科