File System Lab

ext2 file system layout The layout of the system:

Super block:  The superblock contains all the pertinent global information for the entire drive. Information such as: the total number of blocks on disk, the size of the blocks, the number of free blocks, and so forth.

GDT: group descriptor table  The blocks immediately after the superblock contain a list of group descriptors (essentially an array of group descriptors). This list contains a descriptor for each block group.

Block bitmap and Inode bitmap:  ext2 keeps track of free space using bitmaps.  The blocks bitmap tracks free blocks (raw space), and the inode bitmap tracks free inodes.  These bitmaps are fixed size and are exactly 1 block large.  With a block size of 1024, the block bitmap contains = 8192 bits, which means that the block group can have no more than 8K blocks.

Inode table:  An inode is 128byte  file attributes and data blocking information

Data block:  Regular files: Data  Directory: name of files and subdirectory  Symbolic link: name in either inode or a data block

$ dd if=/dev/zero of=fs count=256 bs=4K  if input file  of output file mke2fs fs  To make a file system out of the fs file  Block size; inodes sudo mount –o loop fs /mnt

dumpe2fs fs  Inode count 128  Block count 1024  Free blocks: 986  Free inodes:117  First inode:11  Inode size: 128  Primary superblock: 1  Group descriptor: 2  Block bitmap:6  Inode bitmap: 7  Inode table: 8-23

sudo mount –o loop fs /mnt od -tx1 –Ad fs

Super block od -tx1 –Ad fs –N1024

Group descriptor od –tx1 –Ad fs –N1024 –j2048 Group 0: (Blocks ) Primary superblock at 1, Group descriptors at 2-2 Reserved GDT blocks at 3-5 Block bitmap at 6 (+5), Inode bitmap at 7 (+6) Inode table at 8-23 (+7) 986 free blocks, 117 free inodes, 2 directories Free blocks: Free inodes:

Block bitmap od -tx1 -Ad fs -N1024 -j ff ff ff ff 1f * ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff *

Inode bitmap od -tx1 -Ad fs -N1024 -j ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff *

Inode table od -tx1 -Ad fs –N2048 –j a 2b a 2b a 2b * ed 41 e c9 4b 2b a 2b a 2b e * a 2b a 2b a 2b * * ed a 2b 50 2a 4c 2b a 2b a b c d e

Data block Block 24: od -tx1 -Ad fs -N1024 -j c e c e 2e b e8 03 0a c 6f b 66 6f 75 6e

Data block Block 24: After mkdir a in /mnt c e c e 2e b a c 6f b 66 6f 75 6e c d

Data block Block 24 After touch a.txt c e c e 2e b a c 6f b 66 6f 75 6e c c d c e *

Inode table od -tx1 -Ad fs -N2048 -j ed d3 51 2b 50 e5 51 2b e5 51 2b a b c d e * fd 41 e e9 51 2b 50 e9 51 2b e9 51 2b e * ef 0c 2d b4 81 e b b b e