learning ext2 filesystem notes

reference:  http://e2fsprogs.sourceforge.net/ext2intro.html

reference: http://www.nongnu.org/ext2-doc/index.html

reference: http://www.science.unitn.it/~fiorella/guidelinux/tlk/

Each file is represented by a structure, called an inode. Each inode contains the description of the file: file type, access rights, owners, timestamps, size, pointers to data blocks. The addresses of data blocks allocated to a file are stored in its inode. When a user requests an I/O operation on the file, the kernel code converts the current offset to a block number, uses this number as an index in the block addresses table and reads or writes the physical block.

Directories：

Directories are implemented as a special type of files. Actually, a directory is a file containing a list of entries. Each entry contains an inode number and a file name. When a process uses a pathname, the kernel code searchs in the directories to find the corresponding inode number. After the name has been converted to an inode number, the inode is loaded into memory and is used by subsequent requests.

what happend when a extern stroage was mounted:

When a filesystem is to be mounted, the appropriate mount function is called. This function is responsible for reading the superblock from the disk, initializing its internal variables, and returning a mounted filesystem descriptor to the VFS. After the filesystem is mounted, the VFS functions can use this descriptor to access the physical filesystem routines.

A mounted filesystem descriptor contains several kinds of data: informations that are common to every filesystem types, pointers to functions provided by the physical filesystem kernel code, and private data maintained by the physical filesystem code. The function pointers contained in the filesystem descriptors allow the VFS to access the filesystem internal routines.

Two other types of descriptors are used by the VFS: an inode descriptor and an open file descriptor. Each descriptor contains informations related to files in use and a set of operations provided by the physical filesystem code. While the inode descriptor contains pointers to functions that can be used to act on any file (e.g. create, unlink), the file descriptors contains pointer to functions which can only act on open files (e.g. read, write).

Note:  When such a file is deleted, random data is written in the disk blocks previously allocated to the file. This prevents malicious people from gaining access to the previous content of the file by using a disk editor.

root@IoTP:~# dumpe2fs /dev/mmcblk1p2
dumpe2fs 1.42.13 (17-May-2015)
Filesystem volume name: rootfs
Last mounted on: /
Filesystem UUID: 69277f59-15ba-4705-95eb-b6200ad095dc
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: user_xattr acl
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 236176
Block count: 944384
Reserved block count: 47219
Free blocks: 753105
Free inodes: 204057
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 230
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8144
Inode blocks per group: 509
Filesystem created: Wed Jan 16 05:11:41 2019
Last mount time: Wed Jan 16 06:05:42 2019
Last write time: Wed Jan 16 06:05:42 2019
Mount count: 6
Maximum mount count: -1
Last checked: Wed Jan 16 05:11:41 2019
Check interval: 0 (<none>)
Lifetime writes: 666 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
First orphan inode: 173330
Default directory hash: half_md4
Directory Hash Seed: 32104d28-8e90-4157-9c6a-3a5950fc100a
Journal backup: inode blocks
Journal features: journal_incompat_revoke
Journal size: 64M
Journal length: 16384
Journal sequence: 0x000000a8
Journal start: 9

Group 0: (Blocks 0-32767)
Primary superblock at 0, Group descriptors at 1-1
Reserved GDT blocks at 2-231
Block bitmap at 232 (+232), Inode bitmap at 233 (+233)
Inode table at 234-742 (+234)
8123 free blocks, 8134 free inodes, 1 directories
Free blocks: 752-783, 792-815, 824-847, 856-879, 887-911, 943-975, 1024-1039, 1044-1055, 1057-1073, 1216-1231, 1281-1311, 1327-1359, 1369-1391, 1440-1455, 1497-1503, 1521-1551, 1570-1615, 1618-1647, 1696-1711, 1736-1759, 1780-1807, 1856-1871, 1886-1887, 1898-1935, 1954-1999, 2030-2063, 2094-2127, 2135-2159, 2194-2223, 2228-2255, 2259-2287, 2336-2351, 2356-2367, 2388-2415, 2427-2447, 2484-2511, 2516-2543, 2592-2607, 2725-2751, 2766-2799, 2831-2863, 2870-2895, 2900-2927, 2970-2991, 3022-3055, 3104-3119, 3181-3199, 3214-3247, 3252-3277, 3296-3311, 3328-3347, 3387-3391, 3417-3439, 3455-3514, 3536-3599, 3626-3663, 3712-3791, 3840-3903, 3968-3983, 4124-4223, 4326-4528, 4608-4623, 4813-4847, 5088-5103, 5105-5167, 5200-5215, 5760-5775, 5887, 5910-5935, 5952-5967, 5969-5999, 6291-6303, 6335, 6366-6383, 6385-6418, 6675-6703, 6720-6735, 6757-6799, 6836-6863, 6900-6927, 6962-6991, 7040-7055, 7100-7103, 7136-7151, 7731-7762, 7791-7823, 7872-7887, 7916-7935, 7966-7983, 7988-8015, 8020-8047, 8062-8079, 8159-8175, 8839-8863, 8894-8911, 8933-8964, 9029-9071, 9141-9151, 9183-9199, 9215-9231, 9278-9295, 9344-9359, 9401-9407, 9440-9455, 9536-9551, 9622-9631, 9664-9679, 9744-9759, 9780-9807, 9856-9871, 9936-9951, 9970-9999, 10043-10063, 10098-10127, 10171-10191, 10235-10255, 10296-10319, 10363-10383, 10427-10447, 10496-10511, 10632-10663, 10690-10735, 10784-10799, 10885-10911, 10944-10959, 11002-11007, 11040-11055, 11146-11167, 11200-11215, 11315-11327, 11352-11375, 11404-11439, 11488-11503, 11574-11583, 11616-11631, 11739-11759, 11784-11807, 11840-11855, 12000-12015, 12049-12080, 12145-12159, 12177-12207, 12282-12313, 12368-12399, 12436-12463, 12512-12527, 12603-12623, 12672-12687, 12738-12767, 12800-12815, 12866-12895, 12920-12943, 12978-13007, 13046-13071, 13106-13135, 13179-13199, 13236-13263, 13305-13327, 13376-13391, 13445-13471, 13504-13519, 13647-13679, 13713-13744, 13809-13823, 13829-13871, 13920-13935, 14040-14063, 14088-14111, 14144-14159, 14304-14319, 14341-14372, 14405-14447, 14491-14495, 14528-14543, 14590-14607, 14656-14671, 14726-14751, 14775-14799, 14848-14863, 14900-14911, 14934-14959, 15008-15023, 15123-15135, 15168-15183, 15192-15199, 15235-15279, 15328-15343, 15403-15423, 15442-15471, 15520-15535, 15629-15647, 15672-15695, 15697-15727, 15758-15791, 15822-15855, 15899-15919, 15959-15983, 15993-16015, 16045-16079, 16128-16143, 16170-16191, 16224-16239, 16243-16255, 16320-16335, 16375-16383, 16507-16527, 16534-16559, 16561-16591, 16632-16655, 16660-16687, 16736-16767, 16817-17051, 17132-17151, 17163-17407, 25600-26623, 27329-28671
Free inodes: 11-8144