ext2.txt

嵌入式系统设计与实例开发源码

The Second Extended Filesystem
==============================

ext2 was originally released in January 1993.  Written by R\'emy Card,
Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the
Extended Filesystem.  It is currently still (April 2001) the predominant
filesystem in use by Linux.  There are also implementations available
for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS.

Options
=======

When mounting an ext2 filesystem, the following options are accepted.
Defaults are marked with (*).

bsddf			(*)	Makes `df' act like BSD.
minixdf				Makes `df' act like Minix.

check=none, nocheck	(*)	Don't do extra checking of bitmaps on mount
				(check=normal and check=strict options removed)

debug				Extra debugging information is sent to the
				kernel syslog.  Useful for developers.

errors=continue		(*)	Keep going on a filesystem error.
errors=remount-ro		Remount the filesystem read-only on an error.
errors=panic			Panic and halt the machine if an error occurs.

grpid, bsdgroups		Give objects the same group ID as their parent.
nogrpid, sysvgroups	(*)	New objects have the group ID of their creator.

resuid=n			The user ID which may use the reserved blocks.
resgid=n			The group ID which may use the reserved blocks. 

sb=n				Use alternate superblock at this location.

grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.


Specification
=============

ext2 shares many properties with traditional Unix filesystems.  It has
the concepts of blocks, inodes and directories.  It has space in the
specification for Access Control Lists (ACLs), fragments, undeletion and
compression though these are not yet implemented (some are available as
separate patches).  There is also a versioning mechanism to allow new
features (such as journalling) to be added in a maximally compatible
manner.

Blocks
------

The space in the device or file is split up into blocks.  These are
a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
which is decided when the filesystem is created.  Smaller blocks mean
less wasted space per file, but require slightly more accounting overhead,
and also impose other limits on the size of files and the filesystem.

Block Groups
------------

Blocks are clustered into block groups in order to reduce fragmentation
and minimise the amount of head seeking when reading a large amount
of consecutive data.  Information about each block group is kept in a
descriptor table stored in the block(s) immediately after the superblock.
Two blocks near the start of each group are reserved for the block usage
bitmap and the inode usage bitmap which show which blocks and inodes
are in use.  Since each bitmap is limited to a single block, this means
that the maximum size of a block group is 8 times the size of a block.

The block(s) following the bitmaps in each block group are designated
as the inode table for that block group and the remainder are the data
blocks.  The block allocation algorithm attempts to allocate data blocks
in the same block group as the inode which contains them.

The Superblock
--------------

The superblock contains all the information about the configuration of
the filing system.  The primary copy of the superblock is stored at an
offset of 1024 bytes from the start of the device, and it is essential
to mounting the filesystem.  Since it is so important, backup copies of
the superblock are stored in block groups throughout the filesystem.
The first version of ext2 (revision 0) stores a copy at the start of
every block group, along with backups of the group descriptor block(s).
Because this can consume a considerable amount of space for large
filesystems, later revisions can optionally reduce the number of backup
copies by only putting backups in specific groups (this is the sparse
superblock feature).  The groups chosen are 0, 1 and powers of 3, 5 and 7.

The information in the superblock contains fields such as the total
number of inodes and blocks in the filesystem and how many are free,
how many inodes and blocks are in each block group, when the filesystem
was mounted (and if it was cleanly unmounted), when it was modified,
what version of the filesystem it is (see the Revisions section below)
and which OS created it.

If the filesystem is revision 1 or higher, then there are extra fields,
such as a volume name, a unique identification number, the inode size,
and space for optional filesystem features to store configuration info.

All fields in the superblock (as in all other ext2 structures) are stored
on the disc in little endian format, so a filesystem is portable between
machines without having to know what machine it was created on.

Inodes
------

The inode (index node) is a fundamental concept in the ext2 filesystem.
Each object in the filesystem is represented by an inode.  The inode
structure contains pointers to the filesystem blocks which contain the
data held in the object and all of the metadata about an object except
its name.  The metadata about an object includes the permissions, owner,
group, flags, size, number of blocks used, access time, change time,
modification time, deletion time, number of links, fragments, version
(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).

There are some reserved fields which are currently unused in the inode
structure and several which are overloaded.  One field is reserved for the
directory ACL if the inode is a directory and alternately for the top 32
bits of the file size if the inode is a regular file (allowing file sizes
larger than 2GB).  The translator field is unused under Linux, but is used
by the HURD to reference the inode of a program which will be used to
interpret this object.  Most of the remaining reserved fields have been
used up for both Linux and the HURD for larger owner and group fields,
The HURD also has a larger mode field so it uses another of the remaining
fields to store the extra more bits.

There are pointers to the first 12 blocks which contain the file's data
in the inode.  There is a pointer to an indirect block (which contains
pointers to the next set of blocks), a pointer to a doubly-indirect
block (which contains pointers to indirect blocks) and a pointer to a
trebly-indirect block (which contains pointers to doubly-indirect blocks).

The flags field contains some ext2-specific flags which aren't catered
for by the standard chmod flags.  These flags can be listed with lsattr
and changed with the chattr command, and allow specific filesystem
behaviour on a per-file basis.  There are flags for secure deletion,
undeletable, compression, synchronous updates, immutability, append-only,
dumpable, no-atime, indexed directories, and data-journaling.  Not all
of these are supported yet.

Directories
-----------

A directory is a filesystem object and has an inode just like a file.
It is a specially formatted file containing records which associate
each name with an inode number.  Later revisions of the filesystem also
encode the type of the object (file, directory, symlink, device, fifo,
socket) to avoid the need to check the inode itself for this information
(support for taking advantage of this feature does not yet exist in
Glibc 2.2).

The inode allocation code tries to assign inodes which are in the same
block group as the directory in which they are first created.

The current implementation of ext2 uses a singly-linked list to store
the filenames in the directory; a pending enhancement uses hashing of the
filenames to allow lookup without the need to scan the entire directory.

The current implementation never removes empty directory blocks once they
have been allocated to hold more files.

Special files
-------------

Symbolic links are also filesystem objects with inodes.  They deserve
special mention because the data for them is stored within the inode
itself if the symlink is less than 60 bytes long.  It uses the fields
which would normally be used to store the pointers to data blocks.
This is a worthwhile optimisation as it we avoid allocating a full
block for the symlink, and most symlinks are less than 60 characters long.

Character and block special devices never have data blocks assigned to
them.  Instead, their device number is stored in the inode, again reusing
the fields which would be used to point to the data blocks.

Reserved Space
--------------