Lab 10: Logging and recovery

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Based on the log shown below, answer the following questions.

LSN	transaction	action	item	old value	new value
00	T1	begin
10	T1	update	D1	100	70
20	T2	begin
30	T1	commit
40	T2	update	D1	70	400
60	T2	update	D3	“hello”	“howdy”
70	T3	begin
80	T3	update	D2	15	70
90	T2	commit
100	T3	update	D1	400	55
		*crash*

Recall that we discussed three types of logging: undo-redo, undo-only and redo-only.

How can we can determine from looking at the log that the system is performing undo-redo logging?
At the start of recovery, what are the possible on-disk values of each data item?
If the system were using undo-only logging instead, what are the possible on-disk values of each data item at the start of recovery? You should assume that each data item is on a different page of the database file.
If the system were using redo-only logging instead, what are the possible on-disk values of each data item at the start of recovery? You should assume that each data item is on a different page of the database file.

Consider again the log shown below:

LSN	transaction	action	item	old value	new value
00	T1	begin
10	T1	update	D1	100	70
20	T2	begin
30	T1	commit
40	T2	update	D1	70	400
60	T2	update	D3	“hello”	“howdy”
70	T3	begin
80	T3	update	D2	15	70
90	T2	commit
100	T3	update	D1	400	55
		*crash*

During recovery, what steps would be taken during the first pass through the log if the system is using undo-redo logging?
What steps would be taken during the second pass through the log?
How would the log and the recovery process be different under undo-only logging?
How would the log and the recovery process be different under redo-only logging?

Now assume that the system is performing logical logging, which means:

each on-disk data item must have a corresponding datum LSN, which is the LSN of the log record that produced the value
each update log record must include the old log sequence number (the olsn), as shown below. The olsn is the LSN of the log record that produced the data item’s previous value.

LSN	transaction	action	item	old value	new value	olsn
00	T1	begin
10	T1	update	D1	100	70	0
20	T2	begin
30	T1	commit
40	T2	update	D1	70	400	10
60	T2	update	D3	“hello”	“howdy”	0
70	T3	begin
80	T3	update	D2	15	70	0
90	T2	commit
100	T3	update	D1	400	55	40
		*crash*

What other changes to the log would typically be made under logical logging?
What is the purpose of storing the olsn and datum LSN values?
At the start of recovery, assume that we have the following on-disk datum LSNs:
- D1: 10
- D2: 80
- D3: 60
Trace through the steps taken during the backward and forward passes for the log above.

Now let’s assume that a dynamic checkpoint occurred between log records 40 and 60:

LSN	transaction	action	item	old value	new value
00	T1	begin
10	T1	update	D1	100	70
20	T2	begin
30	T1	commit
40	T2	update	D1	70	400
50		checkpoint
60	T2	update	D3	“hello”	“howdy”
70	T3	begin
80	T3	update	D2	15	70
90	T2	commit
100	T3	update	D1	400	55
		*crash*

What other information must be included in log record 50?
Given the checkpoint, do all of the log records shown need to be examined during the backward pass? During the forward pass? Why or why not?
Does the existence of the checkpoint change the possible on-disk values at the start of recovery under undo-redo logging?

Last updated on December 4, 2024.