Ext2

About ext2fs (Second Extended Filesystem)

The Second Extended Filesystem (ext2fs) was the default filesystem of Linux prior the advent of the journaling file systems ext3fs and ReiserFS. It has native support for UNIX ownership / access rights, symbolic and hard links and other Unix-native properties. Like HPFS, it tries to minimize head movement by distributing data across the disk. Also, by using "groups", it minimizes the impact of fragmentation. It is another "inode" based system. An ext2fs-partition is made up from blocks, which normally are 1K each. The first block (the bootblock) is zeroized, all the other blocks are divided into so-called block groups (normally, between 256 and 8192 blocks form a group). Each block group contains:

• a copy of the superblock (which is a mighty useful structure containing info about the filesystem);
• the filesystem descriptors (dunno what that is exactly)
• the block bitmap, tells which blocks are used
• the inode bitmap, tells which inodes are used (difference?)
• the inode table, which contains the inodes themselves
• the data blocks referenced by the inodes

The first inode is a special one; it is the bad blocks inode, which references all the damaged sectors of the partition. The fifth inode contains the bootloader, whereas the 11th contains the root directory.

Windows users can access ext2fs partitions with explore2fs.

About ext3fs (Third Extended File System)

ext3fs is basically ext2fs with journaling added. If your ext3fs partition does not need journal replay, it can even be accessed with a 'simple' ext2fs driver.

Links

• ext2-doc project: Second Extended File System - implementation-oriented documentation, describes internal structure in human language.
• Design and Implementation of the Second Extended Filesystem (overview)