Lecture: on-disk data structures

preparation

I/O stack
- high-level API: read/write/… using file names and file descriptors
- low-level API: using a block device
  - terminology: block vs sector
  - data ← read(blockno)
  - write(blockno, data)
  - flush (ignore for now)
  - trim (ignore for now)
example: xv6
- per-process fd → struct file → fs layers (fs.c) → block I/O (bio.c) → disk driver (ide.c)
- walkthrough: write() → sys_write() → filewrite() → writei() → bread()/bwrite() → iderw()
  - what does argfd() do in sys_write()?
  - how does filewrite() call different functions depending on the fd type?

questions
- how to locate /d/f: how to locate the root directory, directory d, and file f
- how to append “hello” to /d/f
- how to overwrite /d/f with new data “world”
- how to create a new file /newfile
xv6 disk layout: [ ... | superblock | log | inode blocks | free bitmap | data blocks ]
- ignore log for now
superblock: 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 bitmap: balloc/bfree in fs.c
- track free/in-use data blocks
- bit i in the free bitmap indicates whether data block i is free
- quiz
  - how to check if data block i is free
  - how to change it from free to in-use
inode: 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