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Crash Consistency: FSCK and Journaling

Homework (Simulation)

This section introduces fsck.py, a simple simulator you can use to better understand how file system corruptions can be detected (and potentially repaired). Please see the associated README for details on how to run the simulator.

Questions

1. First, run fsck.py -D; this flag turns off any corruption, and thus you can use it to generate a random file system, and see if you can determine which files and directories are in there. So, go ahead and do that! Use the -p flag to see if you were right. Try this for a few different randomly-generated file systems by setting the seed (-s) to different values, like 1, 2, and 3.

2. Now, let’s introduce a corruption. Run ./fsck.py -S 1 to start. Can you see what inconsistency is introduced? How would you fix it in a real file system repair tool? Use -c to check if you were right.

   Inode bitemap and inode data are inconsistent in bit 13.

3. Change the seed to -S 3 or -S 19; which inconsistency do you see? Use -c to check your answer. What is different in these two cases?

   -S 3: inodes block says file /m has two references, but data block shows it's one.

   -S 19: directory /g has two directories but inode shows it has only one reference.

   The first case may happen when create a link to file /m. But directory must have more than two references.

4. Change the seed to -S 5; which inconsistency do you see? How hard would it be to fix this problem in an automatic way? Use -c to check your answer. Then, introduce a similar inconsistency with -S 38; is this harder/possible to detect? Finally, use -S 642; is this inconsistency detectable? If so, how would you fix the file system?

   -S 5: .. was renamed to link y.

   -S 38: /w/p was renamed to /w/b, it's harder to detect.

   -S 642: /g was renamed to /w, it's detectable. Add a 1 to the folder name.

5. Change the seed to -S 6 or -S 13; which inconsistency do you see? Use -c to check your answer. What is the difference across these two cases? What should the repair tool do when encountering such a situation?

   -S 16: 12th inode doesn't have any references.

   -S 13: 10th inode doesn't have any references.

   The first is a directory, the second is folder. Delete them.

6. Change the seed to -S 9; which inconsistency do you see? Use -c to check your answer. Which piece of information should a check-and-repair tool trust in this case?

   File /m was changed to a directory. Data blocks.

7. Change the seed to -S 15; which inconsistency do you see? Use -c to check your answer. What can a repair tool do in this case? If no repair is possible, how much data is lost?

   / was changed to a file. Change the inode to directory type. All data will be lost.

8. Change the seed to -S 10; which inconsistency do you see? Use -c to check your answer. Is there redundancy in the file system structure here that can help a repair?

   /w was moved to a nonexistent folder. Check data blocks to find which folder has /w then repair the inode.

9. Change the seed to -S 16 and -S 20; which inconsistency do you see? Use -c to check your answer. How should the repair tool fix the problem?

   -S 16: The data block of file \m was changed to an empty block. Delete the file.

   -S 20: The data block of directory \g was changed to an empty block. Search the data blocks, find which may be a directory and . is pointed to \g.