What is a file system?
  A collection of routines in the kernel
    /proc/fs
  User view
    open, creat, read, write, lseek
    mkdir, rmdir
    link, symlink
    chmod, chown
    mount, umount
  An on-disk format
    FAT -- linked lists
    UFS -- trees and pointers
      inodes and directories
    extent-based file systems
      Google for "ext on disk format"
      https://www.usenix.org/legacy/event/lsf07/tech/cao_m.pdf
      https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout
  Mounting file systems
    initiated in user space from files similar to /etc/fstab

What happens when the disk is corrupted?
  What happens if you "lose" a node of a tree or linked list?
  And how did it happen?

Backup strategies

What are you doing the backup
  Looking for the deleted file?
  Restore after last night's crash?
    Perhaps bare metal recovery
    Remember the boot blocks
  Answer that subpoena for discovery
  Clone 50 copies
  Move the database
    May need the database for this one

https://help.ubuntu.com/community/BackupYourSystem

Tools....
  dd   (ugh)
  dump/restore   (too optimized?)
  tar
  rsync
  partclone (clonezilla)
  
Can you really get a consistent copy?
  Suppose you backup in the middle of significant upgrade
    Isn't that like pulling the plug on a running system?
    Will your system boot?
  Getting the quiet file system
    Can you really shut down for a few hours
    Can you take a snapshot?