NIELIT Scientist B 2020 November: 80

Question

Consider a Simple Checkpointing Protocol and the following set of operations in the log.

$(start, T_4);(write, T_4,y,2,3);(start,T_1);$

$(commit,T_4);(write,T_1,z,5,7);$

$(Checkpoint);$

$(start,T_2);(write,T_2,x,1,9);$

$(commit,T_2);(start,T_3);(write,T_3,z,7,2);$

if a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list?

• A. $Undo : T_3,T_1;Redo\;T_2$
• B. $Undo : T_3,T_1;Redo\;T_2,T_4$
• C. $Undo : none; \;Redo\;T_2, T_4, T_3, T_1$
• D. $Undo : T_3,T_1,T_4; \;Redo\;T_2$

Comments

Same Question : https://gateoverflow.in/8246/gate-cse-2015-set-2-question-46

Similar Questions : https://gateoverflow.in/981/gate-cse-2006-question-20-isro2015-17

                               https://gateoverflow.in/357439/gate-cse-2021-set-1-question-13

Answer 1

before check point $T4$ is committed. So it's neither in undo list nor in redo list.

$T1$ is started but not committed before check point, even after check point. So it should be in undo list.

$T2$ is started after check point and committed, So it should be redo list.

$T3$ is started after check point and not committed, so it should be in undo list.

Option $(A)$ is correct.