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 still (April 2001) 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	(*)	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.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.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 (8192 bytes on Alpha systems),which is decided when the filesystem is created.  Smaller blocks meanless 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 fragmentationand minimise the amount of head seeking when reading a large amountof consecutive data.  Information about each block group is kept in adescriptor table stored in the block(s) immediately after the superblock.Two blocks near the start of each group are reserved for the block usagebitmap and the inode usage bitmap which show which blocks and inodesare in use.  Since each bitmap is limited to a single block, this meansthat 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 designatedas the inode table for that block group and the remainder are the datablocks.  The block allocation algorithm attempts to allocate data blocksin the same block group as the inode which contains them.The Superblock--------------The superblock contains all the information about the configuration ofthe filing system.  The primary copy of the superblock is stored at anoffset of 1024 bytes from the start of the device, and it is essentialto mounting the filesystem.  Since it is so important, backup copies ofthe superblock are stored in block groups throughout the filesystem.The first version of ext2 (revision 0) stores a copy at the start ofevery block group, along with backups of the group descriptor block(s).Because this can consume a considerable amount of space for largefilesystems, later revisions can optionally reduce the number of backupcopies by only putting backups in specific groups (this is the sparsesuperblock feature).  The groups chosen are 0, 1 and powers of 3, 5 and 7.The information in the superblock contains fields such as the totalnumber of inodes and blocks in the filesystem and how many are free,how many inodes and blocks are in each block group, when the filesystemwas 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 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 a 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 extended attributes (EAs) and/or Access Control Lists (ACLs).There are some reserved fields which are currently unused in the inodestructure and several which are overloaded.  One field is reserved for thedirectory ACL if the inode is a directory and alternately for the top 32bits of the file size if the inode is a regular file (allowing file sizeslarger than 2GB).  The translator field is unused under Linux, but is usedby the HURD to reference the inode of a program which will be used tointerpret this object.  Most of the remaining reserved fields have beenused 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 remainingfields to store the extra more 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 with lsattrand changed with the chattr command, and allow specific filesystembehaviour 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 allof 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 associateeach name with an inode number.  Later revisions of the filesystem alsoencode 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 inGlibc 2.2).The inode allocation code tries to assign inodes which are in the sameblock group as the directory in which they are first created.The current implementation of ext2 uses a singly-linked list to storethe filenames in the directory; a pending enhancement uses hashing of thefilenames to allow lookup without the need to scan the entire directory.The current implementation never removes empty directory blocks once theyhave been allocated to hold 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 data blocks.This is a worthwhile optimisation as it we avoid allocating a fullblock for the symlink, 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 data blocks.Reserved Space--------------