Ext2 block structure: size of reserved GDT Blocks

In every publication I found about ext2, the structure of a block group is defined as following: - Super Block: 1 block - Group Descriptor: N blocks - Data Bitmap: 1 block - Inode Bitmap: 1 block - Inode Table: N blocks - Data Blocks: remaining blocks However in the ext2 kernel doc it is stated that versions >0 may not store copies of the super block and group descriptors in every block group. When I fsstat my ext2 partition, I get following output: Group: 1: Inode Range: 1977 - 3952 Block Range: 8193 - 16384 Layout: Super Block: 8193 - 8193 Group Descriptor Table: 8194 - 8194 Data bitmap: 8451 - 8451 Inode bitmap: 8452 - 8452 Inode Table: 8453 - 8699 Data Blocks: 8700 - 16384 Free Inodes: 1976 (100%) Free Blocks: 0 (0%) Total Directories: 0 Group: 2: Inode Range: 3953 - 5928 Block Range: 16385 - 24576 Layout: Data bitmap: 16385 - 16385 Inode bitmap: 16386 - 16386 Inode Table: 16387 - 16633 Data Blocks: 16387 - 16386, 16634 - 24576 Free Inodes: 1976 (100%) Free Blocks: 0 (0%) There are two things about this output that confuse me: 1. In groups where the SB and group desc. are stored, there is a gap of 256 blocks between the group desc. and data bitmap. **EDIT: Using dumpe2fs I just found out that these are reserved GDT blocks, used for online resizing .** So the new question is, how is the size of these reserved GDT blocks determined? 2. What does Data Blocks: 16387 - 16386 in Group 2 mean?