Bit set to show block / Inode in use
Bit maps save space
The group bit map is only for blocks in the group
The bit maps are efficient way of showing whether an inode or block is allocated. You can also cross reference the data in the inodes and the bitmaps to see if the system is in a consistent state. This is one of the things that fsck does when it checks your disk. How a bit map works is that for every bit in the map it represents a block on the the disk, so to represents four blocks you would need four bits. This means that every byte in a bit map represents eight blocks. The state of the bit indicates whether the block is free or in use. Each groups bitmap is only for blocks and inodes within the current group.
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