Low Level

• Handles organization of files on disks/media: data structure to find/allocate disk blocks
• File meta data: permissions, owner, times

Higer Level

E.g., Linux VFS (virtual file system)

• Interface for accessing a file system
• Support multiple file systems. Each one implements the vfs interface.

Window supports multiple file systems: e.g., FAT16, FAT32, NTFS

Linux VFS supports multiple file ext2, ext3, ext4, and the Windows file systems.

Linux Interface in C

Each file system has its own FileObject (below) and must provide the functions that are inserted into its FileObject's array of file_ops.

typedef struct {
  unsigned short refcount;
  struct file_ops *file_op;
  /* function pointers */
} FileObject;

Access at higher level is through an array of function pointers. Higher level uses same interface for all file systems, just a different FileObject instance.