3. The Second Extended File system(EXT2)

Filesystem structure:

•       The basic structure is the same for all the different Unix filesystems. The filesystem logically divides the disk into small units calls blocks, a block is the smallest unit which can be allocated.
•       Ext2fs groups together a fixed number of sequential blocks into a group block, and the filesystem is managed as a series of group blocks. As a result, much of the filesystem management reduces to management of a single blocks group. ( another difference between linux ext2fs and filesystems of other operating systems )

Figure 2  Structure of a Unix filesystem(above) and the Ext2 filesystem(below)

Components in ext2fs structure

• Boot block       -- each file system starts with a boot block.
• Superblock
• contain information which describes the state of the internal filesystem
• allow the filesystem manager to use and maintain the file system.
• superblock is located at the fixed offset 1024 in the device, its length is 1024 bytes.

Number of inodes	Number of blocks
Number of reserved blocks	Number of free blocks
Number of free inodes	First data block
Block size	Fragment size
Blocks per group	Fragments per group
Inodes per group	Time of mounting
Time of last write	Status     Max. mount counter
Ext2 signature       Status	Error behavior        Pad word
Time of last test	Max. test interval
Operating system	File system revision
RESUID               RESGID

• Ext2 group descriptor    -- summarizes some necessary information about the group block

 

Block bitmap	Inode bitmap
inode table	No.of free blocks       No.of free inodes
No.of directories

Ext2 Directories

•     In the ext2 file system, directories are implemented in the same way as files are imlemented(with the direct blocks, indirect blocks...)
•     Directories are special files that are used to create and hold access paths to the files in the file system.

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