Lecture: on-disk data structures

recap from last lecture

• xv6’s I/O stack
  • per-process fd → struct file → fs layers (fs.c) → block I/O (bio.c) → disk driver (ide.c)
  • example: sys_write in sysfile.c
    • what does argfd() do?
    • how does filewrite call different functions depending on the fd type?
• block device
  • terminology: block vs sector
  • API
    • data ← read(blockno)
    • write(blockno, data)
    • flush
    • trim

overview

• xv6 disk layout: [ ... | superblock | log | inode blocks | free bit map | data blocks ]
  • ignore log for now
• superblock
  • see struct superblock in fs.h
  • metadata about the entire file system
  • often located at a pre-defined location (xv6: block 1)
  • contain pointers to other parts of the file system.
• free bit map
  • see balloc/bfree in fs.c
  • track free/in-use blocks
• inode
  • see struct dinode in fs.h
  • file metadata: type, size, (permission, time, …)
  • contain pointers to data blocks: 12 direct blocks and 1 (single) indirect block
    • goal: map (inode number, offset) to disk block number
    • why indirect blocks? compare to two-level paging
  • quiz
    • does inode have a “name” field? why not?
    • what’s the maximum file size
    • given a file’s inode, how to read its n-th block - see readi/bmap in fs.c
• file: inode + data blocks
• directory: a special “file”
  • content: struct dirents - (inode number, name) pairs
  • map name to inode number
• questions
  • how to locate /d/f
  • how to write “hello” to /d/f (overwrite vs. append)
  • how to create a new file