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 theExtended Filesystem.  It is currently (February 1999) the predominantfilesystem in use by Linux.  There are also implementations availablefor 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 hasthe concepts of blocks, inodes and directories.  It has space in thespecification for Access Control Lists (ACLs), fragments, undeletion andcompression though these are not yet implemented (some are available asseparate patches).  There is also a versioning mechanism to allow newfeatures (such as journalling) to be added in a maximally compatiblemanner.Blocks------The space in the device or file is split up into blocks.  These area fixed size, of 1024, 2048 or 4096 bytes, which is decided when thefilesystem 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 fragmentationand minimise the amount of head seeking when reading a large amount ofconsecutive data.  Each block group has a descriptor and the array ofdescriptors is stored immediately after the superblock.  Two blocks atthe start of each group are reserved for the block usage bitmap andthe inode usage bitmap which show which blocks and inodes are used.Since each bitmap fits in a block, this means that the maximum size ofa block group is 8 times the size of a block.The first (non-reserved) blocks in the block group are designated asthe inode table for the block and the remainder are the data blocks.The block allocation algorithm attempts to allocate data blocks in thesame block group as the inode which contains them.The Superblock--------------The superblock contains all the information about the configuration ofthe filing system.  It is stored in block 1 of the filesystem (numberingfrom 0) and it is essential to mounting it.  Since it is so important,backup copies of the superblock are stored in block groups throughoutthe filesystem.  The first revision of ext2 stores a copy at the startof every block group.  Later revisions can store a copy in only someblock 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 manyinodes and blocks are in the filesystem and how many are unused, howmany inodes and blocks are in a block group, when the filesystem wasmounted, 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 extrafields, such as a volume name, a unique identifier, the inode size,support for compression, block preallocation and creating fewer backupsuperblocks.All fields in the superblock (as in all other ext2 structures) are storedon the disc in little endian format, so a filesystem is portable betweenmachines 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 inodestructure contains pointers to the filesystem blocks which contain thedata held in the object and all of the metadata about an object exceptits 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 inodestructure and several which are overloaded.  One field is used for thedirectory ACL if the inode is a directory and for the top 32 bits ofthe file size if the inode is a regular file.  The translator field isunused under Linux, but is used by the HURD to reference the inode ofa program which will be used to interpret this object.  The HURD alsohas larger permissions, owner and group fields, so it uses some of theother unused by Linux fields to store the extra bits.There are pointers to the first 12 blocks which contain the file's datain the inode.  There is a pointer to an indirect block (which containspointers to the next set of blocks), a pointer to a doubly-indirectblock (which contains pointers to indirect blocks) and a pointer to atrebly-indirect block (which contains pointers to doubly-indirect blocks).The flags field contains some ext2-specific flags which aren't cateredfor by the standard chmod flags.  These flags can be listed withlsattr and changed with the chattr command.  There are flags for securedeletion, undeletable, compression, synchronous updates, immutability,append-only, dumpable, no-atime, and btree directories.  Not all ofthese 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 associateeach name with an inode number.  Later revisions of the filesystem alsoencode the type of the object (file, directory, symlink, device, fifo,socket) in the directory entry for speed.  The current implementationof ext2 uses a linked list in directories; a planned enhancement willuse btrees instead.  The current implementation also never shrinksdirectories once they have grown to accommodate more files.Special files-------------Symbolic links are also filesystem objects with inodes.  They deservespecial mention because the data for them is stored within the inodeitself if the symlink is less than 60 bytes long.  It uses the fieldswhich would normally be used to store the pointers to blocks to storethe data.  This is a worthwhile optimisation to make as it does not thentake up a block, and most symlinks are less than 60 characters long.Character and block special devices never have data blocks assigned tothem.  Instead, their device number is stored in the inode, again reusingthe fields which would be used to point to the blocks.Revisions---------The revisioning mechanism used in ext2 is sophisticated.  The revisioningmechanism is not supported by version 0 (EXT2_GOOD_OLD_REV) of ext2 butwas introduced in version 1.  There are three 32-bit fields, one forcompatible features, one for read-only compatible features and one forincompatible features.Reserved Space--------------In ext2, there is a mechanism for reserving a certain number of blocksfor a particular user (normally the super-user).  This is intended toallow for the system to continue functioning even if a user fills upall the available space.  It also keeps the filesystem from filling upentirely which helps combat fragmentation.Filesystem check----------------At boot time, most systems run a consistency check (e2fsck) on theirfilesystems.  The superblock of the ext2 filesystem contains severalfields which indicate whether fsck should actually run (since checkingthe filesystem at boot can take a long time if it is large).  fsck willrun if the filesystem was not unmounted without errors, if the maximummount count has been exceeded or if the maximum time between checks hasbeen exceeded.Metadata--------It is frequently claimed that the ext2 implementation of writingasynchronous metadata is faster than the ffs synchronous metadatascheme but less reliable.  Both methods are equally resolvable by theirrespective fsck programs.If you're exceptionally paranoid, there are 3 ways of making metadatawrites 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 +Sper-filesystem: mount -o syncthe first and last are not ext2 specific but do force the metadata tobe written synchronously.References==========The kernel source	file:/usr/src/linux/fs/ext2/Design & Implementation	http://khg.redhat.com/HyperNews/get/fs/ext2intro.htmlCompression		http://debs.fuller.edu/e2compr/ACL support		ftp://tsx-11.mit.edu/pub/linux/ALPHA/ext2fsupdated ACL work	http://aerobee.informatik.uni-bremen.de/acl_eng.htmle2fsprogs 		ftp://tsx-11.mit.edu/pub/linux/packages/ext2fsImplementations 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.htmDOS client		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/RISC OS client		ftp://ftp.barnet.ac.uk/pub/acorn/armlinux/iscafs/