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The Second Extended Filesystem

已有 441 次阅读2011-4-10 07:54 | 关键词:


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
=======
Most defaults are determined by the filesystem superblock, and can be
set using tune2fs(8). Kernel-determined defaults are indicated by (*).
bsddf   (*) Makes `df' act like BSD.
minixdf    Makes `df' act like Minix.
check    Check block and inode bitmaps at mount time
    (requires CONFIG_EXT2_CHECK).
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.
nouid32    Use 16-bit UIDs and GIDs.
oldalloc   Enable the old block allocator. Orlov should
    have better performance, we'd like to get some
    feedback if it's the contrary for you.
orlov   (*) Use the Orlov block allocator.
    (See http://lwn.net/Articles/14633/ and
    http://lwn.net/Articles/14446/.)
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.
user_xattr   Enable "user." POSIX Extended Attributes
    (requires CONFIG_EXT2_FS_XATTR).
    See also http://acl.bestbits.at
nouser_xattr   Don't support "user." extended attributes.
acl    Enable POSIX Access Control Lists support
    (requires CONFIG_EXT2_FS_POSIX_ACL).
    See also http://acl.bestbits.at
noacl    Don't support POSIX ACLs.
nobh    Do not attach buffer_heads to file pagecache.
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
--------------
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).
Check Documentation/filesystems/ext3.txt if you want to read more about
ext3 and journaling.
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/
Compression (*)  http://www.netspace.net.au/~reiter/e2compr/
(*) no longer actively developed/supported (as of Apr 2001)

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