File System Implementation

Disk Layout

┌───────┬───────┬────────┬───────┬────────────┐
│ Boot  │ Super │ Free   │ Inode │   Data     │
│ Block │ Block │ Space  │ Table │   Blocks   │
└───────┴───────┴────────┴───────┴────────────┘

Unix Inode

struct inode {
    mode_t   mode;       // File type & permissions
    uid_t    uid;        // Owner
    gid_t    gid;        // Group
    off_t    size;       // File size
    time_t   atime;      // Access time
    time_t   mtime;      // Modify time
    time_t   ctime;      // Change time
    nlink_t  nlinks;     // Hard link count
    blkcnt_t blocks;     // Block count

    // Block pointers
    block_t  direct[12]; // Direct blocks
    block_t  indirect;   // Single indirect
    block_t  dindirect;  // Double indirect
    block_t  tindirect;  // Triple indirect
};

Common File Systems

File System	OS	Features
ext4	Linux	Journaling, large files
NTFS	Windows	ACLs, encryption, compression
APFS	macOS	Snapshots, clones, encryption
FAT32	Universal	Simple, portable, 4GB limit
exFAT	Universal	Flash drives, large files