ext5 块和节点表

Block and inode Bitmaps 
The data block bitmap tracks the usage of data blocks within the block group. 
The inode bitmap records which entries in the inode table are in use. 
As with most bitmaps, one bit represents the usage status of one data block or inode table entry.  This implies a block group size of 8 * number_of_bytes_in_a_logical_block. 
NOTE: If BLOCK_UNINIT is set for a given block group, various parts of the kernel and e2fsprogs code pretends that the block bitmap contains zeros (i.e. all blocks in the group are free).  However, it is not necessarily the case that no blocks are in use -- if meta_bg is set, the bitmaps and group descriptor live inside the group.  Unfortunately, ext2fs_test_block_bitmap2() will return '0' for those locations, which produces confusing debugfs output.