The included patch updates Documentation/filesystems/ext2 to reflect
current information about ext2. It also adds some more information
that people have told me is hard to find in other places (such as a
description of the superblock compatibility flags, file and filesystem
size limits).
I've updated the parts of the document which refer to patches and
extensions to ext2 to reflect what is currently out there (e.g.
Daniel's hashed/indexed directory code, rather than the btree code,
and Andreas Gruenbacher's EA/ACL code instead of the pure ACL code).
I've checked (and updated) the URLs at the end of the document.
Cheers, Andreas
==========================================================================
--- linux-2.4.4p1.orig/Documentation/filesystems/ext2.txt Sun Oct 1 21:21:20 2000
+++ linux-2.4.4p1-aed/Documentation/filesystems/ext2.txt Fri Apr 20 04:06:45 2001
@@ -4,7 +4,7 @@
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
+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.
@@ -17,11 +17,11 @@
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.
+check=none, nocheck (*) Don't do extra checking of bitmaps on mount
+ (check=normal and check=strict options removed)
-debug For developers only.
+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.
@@ -30,8 +30,8 @@
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.
+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.
@@ -53,46 +53,53 @@
------
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.
+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. 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.
+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. 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.
+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
@@ -101,23 +108,25 @@
Inodes
------
-The inode (index node) is the fundamental concept in the ext2 filesystem.
+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 ACLs.
+(for NFS) and extended attributes (EAs) and/or Access Control Lists (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 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
@@ -126,11 +135,12 @@
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.
+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
-----------
@@ -139,10 +149,19 @@
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.
+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
-------------
@@ -150,31 +169,23 @@
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.
+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 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.
+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 a user fills up
-all the available space. It also keeps the filesystem from filling up
-entirely which helps combat fragmentation.
+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
----------------
@@ -183,9 +194,66 @@
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.
+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
--------
@@ -196,29 +264,98 @@
respective fsck programs.
If you're exceptionally paranoid, there are 3 ways of making metadata
-writes synchronous:
+writes synchronous on ext2:
-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
+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.
+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 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/
-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
+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/
-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/
+
+(*) no longer actively developed/supported (as of Apr 2001)
--
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert
On Fri, 20 Apr 2001, Andreas Dilger wrote:
> The included patch updates Documentation/filesystems/ext2 to reflect
> current information about ext2. It also adds some more information
> that people have told me is hard to find in other places (such as a
> description of the superblock compatibility flags, file and filesystem
> size limits).
To quote Oliver Twist: "Please, Sir, I want some more". How about a
explanation of the significance of GOOD_OLD_REV, etc. In particular, I'm
curious why CURRENT_REV is defined as GOOD_OLD_REV and not DYNAMIC_REV.
--
Daniel
Daniel writes:
> To quote Oliver Twist: "Please, Sir, I want some more". How about a
> explanation of the significance of GOOD_OLD_REV, etc. In particular, I'm
> curious why CURRENT_REV is defined as GOOD_OLD_REV and not DYNAMIC_REV.
One thing that deserves mentioning (related to your question) is the fact
that unused fields in the superblock, inode, and group descriptor table
are always initialized to zero. I believe that this has been true for
most (if not all) of ext2's lifetime.
This means that the s_rev_level field could be assumed to be rev 0 when
it was first assigned (some time between kernel 1.1.48 and 1.1.57,
according to your kernel source browser), and remained such for a
considerable time. The compat fields were added around 1.3.99 - 2.0pre6.
AFAIK, the only difference between a GOOD_OLD_REV filesystem and a
DYNAMIC_REV filesystem is:
- addition of COMPAT flags to superblock
- addition of s_inode_size to superblock (previously fixed at 128 bytes)
- addition of s_first inode to superblock (previously fixed at inode 11)
The s_inode_size and s_first inode fields are still set at the default
values to this day. Since we still have a bit of room in the inode,
and a couple of reserved inodes left, there is still no reason to change
these yet.
The superblock later grew to include the (not very critical) fields:
- addition of s_block_group_number to indicate superblock backup number
- addition of s_uuid, s_volume_name, and s_last_mounted to identify the fs
The rest of the fields are only valid in combination with a COMPAT flag.
The reason why CURRENT_REV is GOOD_OLD_REV and not DYNAMIC_REV is "none".
It isn't really used anywhere. In e2fsck, E2FSCK_CURRENT_REV is 1, and
in libext2fs EXT2_LIB_CURRENT_REV is also 1. Unless we run out of compat
flags or something, I don't see any reason why we would ever want to go
to rev 2.
Cheers, Andreas
--
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert