Consider a system that uses LSN-based operation logging with fuzzy checkpoints and logging of undo’s, as described in the lecture slides and in Chapter 7 of the textbook. Assume that there is no analysis pass before recovery (so there is no dirty page table in the checkpoint record), and that CLR’s (i.e. undo records) are treated as normal updates and do not splice out a portion of the log (as shown on lecture slide 33).
The following sequence of records is found in a database log after a system failure. Data values, such as before and after images, are omitted. The notation P1/r1 means record r1 on page P1.
LSN Trans Operation Type Page/Record Trans backpointer
8 T0 Update P0/r0 null
9 T1 Update P1/r1 null
10 T2 Update P2/r3 null
11 T2 Update P1/r2 10
12 T2 CLR P1/r2 11
13 T1 Update P2/r4 9
14 T2 CLR P2/r3 12
15 T3 Update P0/r5 null
16 checkpoint log record, Active transactions: [T0, 8], [T1,13], [T2, 14], [T3, 15]
17 T0 Commit
18 T2 End Abort
19 T1 Update P3/r6 13
20 T3 Update P3/r7 15
21 T3 CLR P3/r7 20
22 checkpoint log record, Active transactions [T1,19], [T3, 21]
23 T4 Update P2/r3 null
24 T4 Update P0/r8 23
25 T1 Commit
Answer each of the following. In each case, explain briefly why it’s the right answer.
a. Show the log records that must be written by the recovery process, in the proper order, and briefly explain why they must be written. The new log records should have LSNs numbered sequentially starting with 26.
b. What LSN is on each page after recovery?
c. Based on what is in the log, what is the smallest LSN of any log record that might have to be redone?
d. What pages are fetched from disk by the recovery process?
e. Does the log give you enough information to tell whether record-level or page-level lock granularity is being used? If so, which is it and how can you tell? If not, explain why not.
f. Suppose LSN 23 were performed by T3 instead of T4. What does this tell you about the execution of T3?
g. It would be unexpected (i.e., an error) for LSN 24 to update P0/r5. Why?
Now suppose we modify the example so that it uses an analysis pass.
Each checkpoint record now includes a dirty page table as follows:
· In LSN 16, Dirty page table = [P0:8, P1:12, P2:13]
· In LSN 22, Dirty page table = [P3:19]
h. At the time of the second checkpoint, what LSN is on each page on disk?
i. It would be unexpected to find [P0:15] in the dirty page table of LSN 22. Why?
j. Which updates and CLRs may have to be redone?