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Inodes

An inode uniquely describes a file. Here's what an inode looks like on disk:

struct ext2_inode {
  unsigned short i_mode;
  unsigned short i_uid;
  unsigned long  i_size;
  unsigned long  i_atime;
  unsigned long  i_ctime;
  unsigned long  i_mtime;
  unsigned long  i_dtime;
  unsigned short i_gid;
  unsigned short i_links_count;
  unsigned long  i_blocks;
  unsigned long  i_flags;
  unsigned long  i_reserved1;
  unsigned long  i_block[EXT2_N_BLOCKS];
  unsigned long  i_version;
  unsigned long  i_file_acl;
  unsigned long  i_dir_acl;
  unsigned long  i_faddr;
  unsigned char  i_frag;
  unsigned char  i_fsize;
  unsigned short i_pad1;
  unsigned long  i_reserved2[2];
};

i_mode

type of file (character, block, link, etc.) and access rights on the file.

i_uid

uid of the owner of the file.

i_size

logical size in bytes.

i_atime

last time the file was accessed.

i_ctime

last time the inode information of the file was changed.

i_mtime

last time the file content was modified.

i_dtime

when this file was deleted.

i_gid

gid of the file.

i_links_count

number of links pointing to this file.

i_blocks

number of blocks allocated to this file counted in 512 bytes units.

i_flags

flags (see below).

i_reserved1

reserved.

i_block

pointers to blocks (see below).

i_version

version of the file (used by NFS).

i_file_acl

control access list of the file (not used yet).

i_dir_acl

control access list of the directory (not used yet).

i_faddr

block where the fragment of the file resides.

i_frag

number of the fragment in the block.

i_size

size of the fragment.

i_pad1

padding.

i_reserved2

reserved.

As you can see, the inode contains, EXT2_N_BLOCKS (15 in ext2fs 0.5) pointers to block. Of theses pointers, the first EXT2_NDIR_BLOCKS (12) are direct pointers to data. The following entry points to a block of pointers to data (indirect). The following entry points to a block of pointers to blocks of pointers to data (double indirection). The following entry points to a block of pointers to a block of pointers to a block of pointers to data (triple indirection).

The inode flags may take one or more of the following or'ed values:

EXT2_SECRM_FL 0x0001

secure deletion. This usually means that when this flag is set and we delete the file, random data is written in the blocks previously allocated to the file.

EXT2_UNRM_FL 0x0002

undelete. When this flag is set and the file is being deleted, the file system code must store enough information to ensure the undeletion of the file (to a certain extent).

EXT2_COMPR_FL 0x0004

compress file. The content of the file is compressed, the file system code must use compression/decompression algorithms when accessing the data of this file.

EXT2_SYNC_FL 0x0008

synchronous updates. The disk representation of this file must be kept in sync with it's in core representation. Asynchronous I/O on this kind of file is not possible. The synchronous updates only apply to the inode itself and to the indirect blocks. Data blocks are always written asynchronously on the disk.

Some inodes have a special meaning:

EXT2_BAD_INO 1

a file containing the list of bad blocks on the file system.

EXT2_ROOT_INO 2

the root directory of the file system.

EXT2_ACL_IDX_INO 3

ACL inode.

EXT2_ACL_DATA_INO 4

ACL inode.

EXT2_BOOT_LOADER_INO 5

the file containing the boot loader. (Not used yet it seems.)

EXT2_UNDEL_DIR_INO 6

the undelete directory of the system.

EXT2_FIRST_INO 11

this is the first inode that does not have a special meaning.

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