ext2.txt

嵌入式系统设计与实例开发源码

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 non-priveleged users
fill up all the space available to them (this is independent of filesystem
quotas).  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 cleanly unmounted, if the maximum mount
count has been exceeded or if the maximum time between checks has been
exceeded.

Feature Compatibility
---------------------

The compatibility feature mechanism used in ext2 is sophisticated.
It safely allows features to be added to the filesystem, without
unnecessarily sacrificing compatibility with older versions of the
filesystem code.  The feature compatibility mechanism is not supported by
the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
revision 1.  There are three 32-bit fields, one for compatible features
(COMPAT), one for read-only compatible (RO_COMPAT) features and one for
incompatible (INCOMPAT) features.

These feature flags have specific meanings for the kernel as follows:

A COMPAT flag indicates that a feature is present in the filesystem,
but the on-disk format is 100% compatible with older on-disk formats, so
a kernel which didn't know anything about this feature could read/write
the filesystem without any chance of corrupting the filesystem (or even
making it inconsistent).  This is essentially just a flag which says
"this filesystem has a (hidden) feature" that the kernel or e2fsck may
want to be aware of (more on e2fsck and feature flags later).  The ext3
HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
a regular file with data blocks in it so the kernel does not need to
take any special notice of it if it doesn't understand ext3 journaling.

An RO_COMPAT flag indicates that the on-disk format is 100% compatible
with older on-disk formats for reading (i.e. the feature does not change
the visible on-disk format).  However, an old kernel writing to such a
filesystem would/could corrupt the filesystem, so this is prevented. The
most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
sparse groups allow file data blocks where superblock/group descriptor
backups used to live, and ext2_free_blocks() refuses to free these blocks,
which would leading to inconsistent bitmaps.  An old kernel would also
get an error if it tried to free a series of blocks which crossed a group
boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.

An INCOMPAT flag indicates the on-disk format has changed in some
way that makes it unreadable by older kernels, or would otherwise
cause a problem if an old kernel tried to mount it.  FILETYPE is an
INCOMPAT flag because older kernels would think a filename was longer
than 256 characters, which would lead to corrupt directory listings.
The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
doesn't understand compression, you would just get garbage back from
read() instead of it automatically decompressing your data.  The ext3
RECOVER flag is needed to prevent a kernel which does not understand the
ext3 journal from mounting the filesystem without replaying the journal.

For e2fsck, it needs to be more strict with the handling of these
flags than the kernel.  If it doesn't understand ANY of the COMPAT,
RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
because it has no way of verifying whether a given feature is valid
or not.  Allowing e2fsck to succeed on a filesystem with an unknown
feature is a false sense of security for the user.  Refusing to check
a filesystem with unknown features is a good incentive for the user to
update to the latest e2fsck.  This also means that anyone adding feature
flags to ext2 also needs to update e2fsck to verify these features.

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 on ext2:

per-file if you have the program source: use the O_SYNC flag to open()
per-file if you don't have the source: use "chattr +S" on the file
per-filesystem: add the "sync" option to mount (or in /etc/fstab)

the first and last are not ext2 specific but do force the metadata to
be written synchronously.  See also Journaling below.

Limitations
-----------

There are various limits imposed by the on-disk layout of ext2.  Other
limits are imposed by the current implementation of the kernel code.
Many of the limits are determined at the time the filesystem is first
created, and depend upon the block size chosen.  The ratio of inodes to
data blocks is fixed at filesystem creation time, so the only way to
increase the number of inodes is to increase the size of the filesystem.
No tools currently exist which can change the ratio of inodes to blocks.

Most of these limits could be overcome with slight changes in the on-disk
format and using a compatibility flag to signal the format change (at
the expense of some compatibility).

Filesystem block size:     1kB        2kB        4kB        8kB

File size limit:          16GB      256GB     2048GB     2048GB
Filesystem size limit:  2047GB     8192GB    16384GB    32768GB

There is a 2.4 kernel limit of 2048GB for a single block device, so no
filesystem larger than that can be created at this time.  There is also
an upper limit on the block size imposed by the page size of the kernel,
so 8kB blocks are only allowed on Alpha systems (and other architectures
which support larger pages).

There is an upper limit of 32768 subdirectories in a single directory.

There is a "soft" upper limit of about 10-15k files in a single directory
with the current linear linked-list directory implementation.  This limit
stems from performance problems when creating and deleting (and also
finding) files in such large directories.  Using a hashed directory index
(under development) allows 100k-1M+ files in a single directory without
performance problems (although RAM size becomes an issue at this point).

The (meaningless) absolute upper limit of files in a single directory
(imposed by the file size, the realistic limit is obviously much less)
is over 130 trillion files.  It would be higher except there are not
enough 4-character names to make up unique directory entries, so they
have to be 8 character filenames, even then we are fairly close to
running out of unique filenames.

Journaling
----------

A journaling extension to the ext2 code has been developed by Stephen
Tweedie.  It avoids the risks of metadata corruption and the need to
wait for e2fsck to complete after a crash, without requiring a change
to the on-disk ext2 layout.  In a nutshell, the journal is a regular
file which stores whole metadata (and optionally data) blocks that have
been modified, prior to writing them into the filesystem.  This means
it is possible to add a journal to an existing ext2 filesystem without
the need for data conversion.

When changes to the filesystem (e.g. a file is renamed) they are stored in
a transaction in the journal and can either be complete or incomplete at
the time of a crash.  If a transaction is complete at the time of a crash
(or in the normal case where the system does not crash), then any blocks
in that transaction are guaranteed to represent a valid filesystem state,
and are copied into the filesystem.  If a transaction is incomplete at
the time of the crash, then there is no guarantee of consistency for
the blocks in that transaction so they are discarded (which means any
filesystem changes they represent are also lost).

The ext3 code is currently (Apr 2001) available for 2.2 kernels only,
and not yet available for 2.4 kernels.

References
==========

The kernel source	file:/usr/src/linux/fs/ext2/
e2fsprogs (e2fsck)	http://e2fsprogs.sourceforge.net/
Design & Implementation	http://e2fsprogs.sourceforge.net/ext2intro.html
Journaling (ext3)	ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
Hashed Directories	http://kernelnewbies.org/~phillips/htree/
Filesystem Resizing	http://ext2resize.sourceforge.net/
Extended Attributes &
Access Control Lists	http://acl.bestbits.at/
Compression (*)		http://www.netspace.net.au/~reiter/e2compr/

Implementations for:
Windows 95/98/NT/2000	http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.htm
Windows 95 (*)		http://www.yipton.demon.co.uk/content.html#FSDEXT2
DOS client (*)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
OS/2			http://perso.wanadoo.fr/matthieu.willm/ext2-os2/
RISC OS client		ftp://ftp.barnet.ac.uk/pub/acorn/armlinux/iscafs/

(*) no longer actively developed/supported (as of Apr 2001)