[linuxkernelnewbies] Analysis of the Ext2fs structure - Directories

Tue, 21 Oct 2008 01:04:28 -0700

http://www.nondot.org/sabre/os/files/FileSystems/ext2fs/ext2fs_8.html#SEC8

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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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Allocation algorithms

Here are the allocation algorithms that ext2 file system managers must use. We are adamant on this point. Nowadays, many users use more than one operating system on the same computer. If more than one operating system use the same ext2 partition, they have to use the same allocation algorithms. If they do otherwise, what will happen is that one file system manager will undo the work of the other file system manager. It is useless to have a manager that uses highly efficient allocation algorithms if the other one does not bother with allocation and uses quick and dirty algorithms.

Here are the rules used to allocate new inodes:

  • the inode for a new file is allocated in the same group of the inode of its parent directory.

  • inodes are allocated equally between groups.

Here are the rules used to allocate new blocks:

  • a new block is allocated in the same group as its inode.

  • allocate consecutive sequences of blocks.

Of course, it may be sometimes impossible to abide by those rules. In this case, the manager may allocate the block or inode anywhere.

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Error Handling

This chapter describes how a standard ext2 file system must handle errors. The superblock contains two parameters controlling the way errors are handled. See section Superblock

The first of these is the s_mount_opt member of the superblock structure in memory. Its value is computed from the options specified when the fs is mounted. Its error handling related values are:

EXT2_MOUNT_ERRORS_CONT
continue even if an error occurs.

EXT2_MOUNT_ERRORS_RO
remount the file system read only.

EXT2_MOUNT_ERRORS_PANIC
the kernel panics on error.

The second of these is the s_errors member of the superblock structure on disk. It may take one of the following values:

EXT2_ERRORS_CONTINUE
continue even if an error occurs.

EXT2_ERRORS_RO
remount the file system read only.

EXT2_ERRORS_PANIC
in which case the kernel simply panics.

EXT2_ERRORS_DEFAULT
use the default behavior (as of 0.5a EXT2_ERRORS_CONTINUE).

s_mount_opt has precedence on s_errors.

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