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1. Crash Consistency

Initial file system, one file, one block: B[v1], I[v1], Da

Data block Db appended, requires 3 writes: B[v2], I[v2], Db

Crash Scenarios:

1. B[v2] written, but not I[v2] and Db

2. I[v2] written, but not B[v2] and Db

3. Db written, but not B[v2] and I[v2]

4. B[v2] and I[v2] written, but not Db

5. I[v2] and Db written, but not B[v2]

6. B[v2] and Db written, but not I[v2]

Which leave file system inconsistent?

How to fix?

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2. fsck

File System Checks:

• Superblock

• Free blocks: inode and data bitmaps

• Inode state

• Inode link counts

• Duplicates

• Bad blocks

• Directory consistency

Slow.

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3. Data Journaling

Write-ahead logging

Without journal:

With journal:

Journal:

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4. Exercises

Using fsck.py:

$ python ./fsck.py -n 6 -i 8 -d 8 -c

Initial state of file system:

             01234567
inode bitmap 10001101

             0           1  2  3  4           5            6  7
inodes       [d a:0 r:3] [] [] [] [d a:3 r:2] [f a:-1 r:1] [] [f a:6 r:1]

             01234567
data bitmap  10010010

             0                          1  2  3                    4  5  6   7
data         [(.,0) (..,0) (g,4) (t,7)] [] [] [(.,4) (..,0) (x,5)] [] [] [z] []


--> determine which files and directories are there


Final state of file system:

             01234567
inode bitmap 10001101

             0           1  2  3  4           5            6  7
inodes       [d a:0 r:3] [] [] [] [d a:4 r:2] [f a:-1 r:1] [] [f a:6 r:1]

             01234567
data bitmap  10010010

             0                          1  2  3                    4  5  6   7
data         [(.,0) (..,0) (g,4) (t,7)] [] [] [(.,4) (..,0) (x,5)] [] [] [z] []


--> CORRUPTION = ?