> HW10
> released: 04/21, 9:00
> due: 04/27, 23:59
> 
> Answer the following questions.
> Submit your answers to Canvas assignments. There is an entry for this homework.
> 
> 
> 1. Redo logging (4 points)
> 
> Read notes Week14b about the five steps of redo logging.
> fs was running one transaction, mkdir("/tmp/a", 0777), and then crashed.
> 
> Questions:
> 
>  1.a The crash happened when mkdir was doing step3 ("write to journal").
>      Later, fs finishes a recovery. Does "/tmp/a" exist in fs? Why?
> 

    [answer:
      "/tmp/a" will not appear because according to redo recovery, fs ignores
      the uncommitted transaction (and mkdir haven't gone to step4 commit).
    ]

> 
>  1.b The crash happened when mkdir was doing step5 ("checkpointing").
>      Later, fs finishes a recovery. Does "/tmp/a" exist in fs? Why?
> 
    [answer:
      "/tmp/a" will appear because according to redo recovery, fs will rerun
      all the updates in the committed transaction. The "/tmp/a" will appear no
      matter what.
    ]
> 
> 
> 2. Undo logging (4 points)
> 
> Read notes Week14b about the four steps of undo logging.
> 
> Questions:
> 
>   2.a Does step2 have to wait for step1? Why?
> 
    [answer:
        yes. otherwise, fs might update some data blocks (in step2)
        before having the begin txn in journal (step1).
        This is a bummer because fs doesn't recognize this uncommitted
        transaction (no begin txn event) hence will not undo the data blocks.
    ]
> 
>   2.b Does step4 have to wait for step3? Why?
> 
    [answer:
        yes. otherwise, when fs crashes after step4 without finishing step3, fs
        will not roll back the unfinished transaction, which leaves
        inconsistency in disk (because step3 is half-done).
    ]
> 
> 
> 3. Authentication (1 point)
> 
> Please write down an example of authentication in real-world non-CS scenarios.
> 
> 
  [answer:
   One example is entering some northeastern buildings: they need to verify
   you're a student of northeastern; and you need to show them your student id.
  ]
> 
> 
> 4. Feedback (1 point)
> 
> This is to gather feedback. Any answer, except a blank one, will get the piont.
> 
>   3.a  Please state the topic or topics in this class that have been least clear to you.
> 
>   3.b  Please state the topic or topics in this class that have been most clear to you.
>