Lecture: file systems

preparation

• read OSPP §11–§13

administrivia

• lab 6 is out
• lab X: browse all the challenge problems in labs 1–6 and project ideas & talk to us
• check SETGATE for possible races

overview

• file system goals
  • data persistence across reboots
  • resource naming & sharing
• user-space programming interfaces
  • review CSE 333 on low-level I/O
  • file-system syscalls: open/close/read/write/fsync/link/unlink/…
  • mmap syscalls: memory rw/mmap/munmap/msync
  • others: async I/O (e.g., Windows)
• questions
  • why fd? how about syscalls using file names only?
  • given a fd, can you get the corresponding file name?
  • can multiple directories “contain” the same file?
  • what happens on disk after running the following code? you should be able to describe the details after this week

int fd = open("/d/f", O_RDWR | O_CREAT, 0644);
write(fd, "hello", 5);
close(fd);

I/O stacks

• xv6
  • per-process fd → struct file → fs layers (fs.c) → block I/O (bio.c) → disk driver (ide.c)
• real-world I/O stack: Linux

block devices

• storage medium
  • tape
  • floppy disk
  • HDD - hard disk drive
  • SSD - solid-state drive
  • others:
    • 3D XPoint
    • blu-ray w/ robotic arms
    • DNA
• disk controller
  • connect storage devices to the bus
  • may perform virtual-to-physical translation (e.g., Flash Translation Layer)
  • examples
    • NVM Express: driver in JOS lab 5
    • PATA (IDE): driver in bootloader & xv6
    • SATA/AHCI
    • others: VirtIO, SCSI - consider lab X
    • read specs & write drivers: see JOS’s kern/nvmereg.h
• device abstraction: block device