Memory

• just like RAM, hard drives provide us with a contiguous stretch of memory where we can store data.
• information in RAM is byte-addressable, meaning that each byte has a unique address.
• similarly, information on a hard drive is sector-addressable, must read or write an entire sector at a time.
• sectors are often 512 bytes in size, but not always.

Readwrite

• API might look like this:

class Drive {
public:
  size_t getSectorSize() const;
  void readSector(size_t num, unsigned char data[]) const;
  void writeSector(size_t num, const unsigned char data[]);
};

Block vs Sector

• Sectors are the smallest unit of storage on a hard drive, they are the physical storage units on the hard drive.
• Blocks are the smallest unit of storage that the filesystem can address and operate.

Filesystem

file metadata

• boot block: contains the bootstrap code that the computer executes when it boots up.
• superblock: contains metadata about the filesystem.
• inodes: includes the size of the filesystem, the size of the blocks, and the number of blocks in the filesystem.

file contents

• file payloads are stored in data blocks.

The inode

• In Unix, the inode is a data structure that stores information about a file to track which blocks are associated with that file.
• inodes are 32-byte data structures that contain metadata about files.
  • stored within an inode: file size, permissions, timestamps, and pointers to the blocks that store the file's data.
• a file's inodes tell us where the file's data blocks are stored on the disk, but the inode also has to be stored somewhere.

struct inode {
  uint16_6 i_mode; // file type and permissions
  uint8_t i_nlink; // number of hard links to the file
  uint8_t i_uid; // user ID of the file's owner
  uint8_t i_gid; // group ID of the file's group
  uint8_t i_size0; // most significant bits of the file size
  uint16_t i_size1; // least significant bits of the file size
  uint16  uint16_t i_addr[8]; // block numbers for the file's data
  uint16_t i_atime[2]; // last access time
  uint16_t i_mtime[2]; // last modification time
};

• we rely on file names to access files, but the computer uses inode numbers to access files.

Addressing file