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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 (February 1999) 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 Perform no checks upon the filesystem.
check=normal (*) Perform normal checks on the filesystem.
check=strict Perform extra checks on the filesystem.
debug For developers only.
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 which may use the reserved blocks.
resgid=n The group 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, which is decided when the
filesystem is created. Smaller blocks mean less wasted space per file,
but require slightly more accounting overhead.
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. Each block group has a descriptor and the array of
descriptors is stored immediately after the superblock. Two blocks at
the start of each group are reserved for the block usage bitmap and
the inode usage bitmap which show which blocks and inodes are used.
Since each bitmap fits in a block, this means that the maximum size of
a block group is 8 times the size of a block.
The first (non-reserved) blocks in the block group are designated as
the inode table for the block 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. It is stored in block 1 of the filesystem (numbering
from 0) and it is essential to mounting it. Since it is so important,
backup copies of the superblock are stored in block groups throughout
the filesystem. The first revision of ext2 stores a copy at the start
of every block group. Later revisions can store a copy in only some
block groups to reduce the amount of redundancy on large filesystems.
The groups chosen are 0, 1 and powers of 3, 5 and 7.
The information in the superblock contains fields such as how many
inodes and blocks are in the filesystem and how many are unused, how
many inodes and blocks are in a block group, when the filesystem was
mounted, when it was modified, what version of the filesystem it is
(see the Revisions section below) and which OS created it.
If the revision of the filesystem is recent enough then there are extra
fields, such as a volume name, a unique identifier, the inode size,
support for compression, block preallocation and creating fewer backup
superblocks.
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 the 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 ACLs.
There are several reserved fields which are currently unused in the inode
structure and several which are overloaded. One field is used for the
directory ACL if the inode is a directory and for the top 32 bits of
the file size if the inode is a regular file. 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. The HURD also
has larger permissions, owner and group fields, so it uses some of the
other unused by Linux fields to store the extra 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. There are flags for secure
deletion, undeletable, compression, synchronous updates, immutability,
append-only, dumpable, no-atime, and btree directories. 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) in the directory entry for speed. The current implementation
of ext2 uses a linked list in directories; a planned enhancement will
use btrees instead. The current implementation also never shrinks
directories once they have grown to accommodate 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 blocks to store
the data. This is a worthwhile optimisation to make as it does not then
take up a block, 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 blocks.
Revisions
---------
The revisioning mechanism used in ext2 is sophisticated. The revisioning
mechanism is not supported by version 0 (EXT2_GOOD_OLD_REV) of ext2 but
was introduced in version 1. There are three 32-bit fields, one for
compatible features, one for read-only compatible features and one for
incompatible features.
Reserved Space
--------------
In ext2, there is a mechanism for reserving a certain number of blocks
for a particular user (normally the super-user). This is intended to
allow for the system to continue functioning even if a user fills up
all the available space. It also keeps the filesystem from filling up
entirely which helps combat fragmentation.
Filesystem check
----------------
At boot time, most systems run a consistency check (e2fsck) on their
filesystems. The superblock of the ext2 filesystem contains several
fields which indicate whether fsck should actually run (since checking
the filesystem at boot can take a long time if it is large). fsck will
run if the filesystem was not unmounted without errors, if the maximum
mount count has been exceeded or if the maximum time between checks has
been exceeded.
Metadata
--------
It is frequently claimed that the ext2 implementation of writing
asynchronous metadata is faster than the ffs synchronous metadata
scheme but less reliable. Both methods are equally resolvable by their
respective fsck programs.
If you're exceptionally paranoid, there are 3 ways of making metadata
writes synchronous:
per-file if you have the source: use the O_SYNC argument to open()
per-file if you don't have the source: use chattr +S
per-filesystem: mount -o sync
the first and last are not ext2 specific but do force the metadata to
be written synchronously.
References
==========
The kernel source file:/usr/src/linux/fs/ext2/
Design & Implementation http://khg.redhat.com/HyperNews/get/fs/ext2intro.html
Compression http://debs.fuller.edu/e2compr/
ACL support ftp://tsx-11.mit.edu/pub/linux/ALPHA/ext2fs
updated ACL work http://aerobee.informatik.uni-bremen.de/acl_eng.html
e2fsprogs ftp://tsx-11.mit.edu/pub/linux/packages/ext2fs
Implementations for:
OS/2 http://perso.wanadoo.fr/matthieu.willm/ext2-os2/
Windows 95 http://www.yipton.demon.co.uk/
Windows NT http://www.cyco.nl/~andreys/ext2fsnt/
http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.htm
DOS client ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
RISC OS client ftp://ftp.barnet.ac.uk/pub/acorn/armlinux/iscafs/
|
