Crash Recovery Vu Tuyet Trinh trinhvt@it-hut.edu.vn Department of Information Systems, Faculty of Information Technology Hanoi University of Technology Transaction collection of action that preserve consistency Consistent DB T Consistent DB’ with assumption IF THEN T starts with consistent state + T executes in isolation T leaves consistent state fb.com/groups/ 1 How can constraints be violated?    Transaction bug DBMS bug Hardware failure e.g., disk crash  Data sharing e.g., T1 and T2 in parallel Failures Events Desired Undesired Expected Unexpected processor CPU memory disk M D fb.com/groups/ 2 Recovery  Maintaining the consistency of DB by ROLLBACK to the last consistency state.  Ensuring 2 properties   Atomic  Durability Using LOG Transaction Log  A sequence of log record keeping trace of actions executed by DBMS <start T> Log the beginning of the transaction execution <commit T> transaction is already finished <abort T> Transaction is calcel <T, X, v, w> Transaction makes an update actio, before update X=v, after update x = w fb.com/groups/ 3 Transaction Log  Handled in main memory and put to external memory (disk) when possible A = 8 16 B = 8 16 Actions Data Log Log Disk Memory Checkpoint  Definition:    Objective    moment where intermediate results and a log record are saved to disk. being initiated at specified intervals minimize the amount of time and effort wasted when restart the process can be restarted from the latest checkpoint rather than from the beginning. Log record <checkpoint> or <ckpt> fb.com/groups/ 4 Undo-logging Step Action t Mem A Read(A,t) t:=t*2 Write(A,t) 8 8 16 16 Mem B Disk A Disk B 8 8 8 8 8 8 8 8 8 1 <start T> 5 Read(B,t) 8 16 16 8 6 t:=t*2 16 16 8 8 8 7 16 16 16 8 8 8 Write(B,t) Flush log 9 Output(A) 16 16 16 16 8 10 Output(B) 16 16 16 16 16