28. File System Structure

A file system is the layer between open("file.txt") and actual bits on disk.

+----------------------+   Block 0
|      Boot Block      |
+----------------------+
|      Superblock      |
+----------------------+
|     Inode Bitmap     |
+----------------------+
|     Data Bitmap      |
+----------------------+
|      Inode Table     |
+----------------------+
|      Data Blocks     |
+----------------------+

• Boot Block: first block on disk; holds bootloader code. Every disk has one, even non-bootable ones.
• Superblock: filesystem-wide metadata — total inodes/blocks, free counts, filesystem type, block size. Corruption here can lose the whole filesystem, so OSes keep backup copies.
• Inode Bitmap / Data Bitmap: one bit per inode / per data block, marking used vs free.
• Inode Table: the array of all inodes, fixed size, created at format time.
• Data Blocks: the actual file contents (directories are stored here too, as special files mapping names to inode numbers).

Common filesystems: ext4 (Linux, journaling), NTFS (Windows, permissions + journaling), APFS (macOS, snapshots + encryption), FAT32 (universal, simple, no permissions), XFS (Linux, high performance for large files).

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