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diff --git a/Documentation/filesystems/ext2.txt b/Documentation/filesystems/ext2.txt deleted file mode 100644 index 94c2cf0292f5..000000000000 --- a/Documentation/filesystems/ext2.txt +++ /dev/null @@ -1,388 +0,0 @@ - -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=none, nocheck (*) Don't do extra checking of bitmaps on mount - (check=normal and check=strict options removed) - -dax Use direct access (no page cache). See - Documentation/filesystems/dax.txt. - -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). -nouser_xattr Don't support "user." extended attributes. - -acl Enable POSIX Access Control Lists support - (requires CONFIG_EXT2_FS_POSIX_ACL). -noacl Don't support POSIX ACLs. - -nobh Do not attach buffer_heads to file pagecache. - -quota, usrquota Enable user disk quota support - (requires CONFIG_QUOTA). - -grpquota Enable group disk quota support - (requires CONFIG_QUOTA). - -noquota option ls 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-privileged 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 32000 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/ext4/ if you want to read more about -ext4 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/ -Filesystem Resizing http://ext2resize.sourceforge.net/ -Compression (*) http://e2compr.sourceforge.net/ - -Implementations for: -Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs -Windows 95 (*) http://www.yipton.net/content.html#FSDEXT2 -DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ -OS/2 (+) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ -RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ - -(*) no longer actively developed/supported (as of Apr 2001) -(+) no longer actively developed/supported (as of Mar 2009) |