Bài giảng Thiết kế và quản trị cơ sở dữ liệu - Crash recovery - Vũ Tuyết Trinh
Crash Recovery
Vu Tuyet Trinh
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
T starts with consistent state +
T executes in isolation
THEN
T leaves consistent state
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
M
memory
disk
D
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
3
Transaction Log
Handled in main memory and put to external
memory (disk) when possible
A = 8 16
B = 8 16
Data
Log
Actions
Log
Disk
Memory
Checkpoint
Definition:
moment where intermediate results and a log record are saved
to disk.
being initiated at specified intervals
Objective
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>
4
Undo-logging
Step
Action
t
Mem A Mem B Disk A Disk B Mem Log
1
2
<start T>
Read(A,t)
t:=t*2
8
16
16
8
8
8
8
8
8
8
8
8
8
8
3
Write(A,t)
Read(B,t)
t:=t*2
16
16
4
<T, A, 8>
<T, B, 8>
8
8
5
6
16
16
16
16
8
8
8
8
7
Write(B,t)
Flush log
Output(A)
Output(B)
16
8
9
16
16
16
16
16
16
16
16
8
10
11
12
16
<commit T>
Flush log
Undo-Logging Rules
(1) For every action generate undo log record (containing
old value)
(2) Before X is modified on disk, log records pertaining to X
must be on disk (write ahead logging: WAL)
(3) Before commit is flushed to log, all writes of transaction
must be reflected on disk
5
Undo Logging Recovery Rules
Let S is set of unfinished transactions
<start Ti> in log
<commit Ti> or <abort Ti> is not in log
For each <Ti, X, v> in log
If Ti S
then
- Write(X, v)
- Output(X)
For each Ti S
Write <abort Ti> to log
Undo-Logging & Checkpoint
<start T1>
<start T1>
<T1, A, 5>
<T1, A, 5>
<start T2>
<start T2>
<T2, B, 10>
<start ckpt (T1,T2)>
<T2, B, 10>
<T2, C, 15>
<T2, D, 20>
<commit T1>
<commit T2>
<checkpoint>
<start T3>
<T2, C, 15>
<start T3>
<T1, D, 20>
<commit T1>
<T3, E, 25>
<commit T2>
<end ckpt>
<T3, F, 30>
<T3, E, 25>
<T3, F, 30>
scan
scan
6
Redo-logging
Step Action
t
Mem A Mem B Disk A Disk B Mem Log
1
<start T>
2
3
4
5
6
7
8
9
Read(A,t)
8
16
16
8
8
8
8
8
8
8
8
8
8
8
t:=t*2
Write(A,t)
Read(B,t)
t:=t*2
16
16
<T, A, 16>
8
8
16
16
16
16
8
8
8
8
Write(B,t)
16
<T, B, 16>
<commit T>
Flush log
10 Output(A) 16
11
Output(B) 16
16
16
16
16
16
16
8
16
<T, end>
Redo-logging Rules
(1) For every action, generate redo log record (containing
new value)
(2) Before X is modified on disk (DB),all log records for
transaction that modified X (including commit) must be
on disk
(3) Flush log at commit
(4) Write END record after DB updates flushed to disk
7
Redo-logging Recovery Rules
Let S = set of transactions with
<Ti, commit> in log
no <Ti, end> in log
For each <Ti, X, v> in log, in forward order (earliest
latest)
If Ti S then write(X, v)
output(X)
For each Ti S
write <Ti, end>
Redo Logging & Checkpoint
<start T1>
<start T1>
<T1, A, 5>
<T1, A, 5>
<start T2>
<start T2>
<commit T1>
<commit T1>
<T2, B, 10>
<T2, B, 10>
<start ckpt (T2)>
<start ckpt (T2)>
<T2, C, 15>
<start T3>
<T3, D, 20>
scan
<T2, C, 15>
<start T3>
<T3, D, 20>
<end ckpt>
<commit T2>
<commit T3>
scan
8
Discussion
Undo Logging
need to write to disk as soon transaction finishes
Access disk
Redo Logging
need to keep all modified blocks in memory until commit
Use memory
Undo/Redo Loggin
Step Action
t
Mem A Mem B Disk A Disk B
Mem Log
<start T>
1
2
3
Read(A,t)
8
16
16
8
8
8
8
8
8
8
8
8
8
8
t:=t*2
Write(A,t)
Read(B,t)
t:=t*2
16
16
4
<T, A, 8, 16>
<T, B, 8, 16>
<commit T>
8
8
5
6
16
16
16
16
8
8
8
8
7
Write(B,t)
Flush log
Output(A)
16
8
9
16
16
16
16
16
16
16
16
8
10
11
Output(B)
16
9
Undo/Redo Logging Rules
Page X can be flushed before or after T commit
Log record flushed before corresponding
updated page (WAL)
Flush at commit (log only)
Undo/Redo Logging & Checkpoint
<start T1>
<T1, A, 4, 5>
<start T2>
<commit T1>
<start T1>
<T1, A, 4, 5>
<start T2>
<start T3>
<commit T1>
<T2, B, 9, 10>
<T3, E, 6, 7>
<start ckpt (T2, T3)>
<T2, C, 14, 15>
<T3, D, 19, 20>
<end ckpt>
<T2, B, 9, 10>
<start ckpt (T2)>
<T2, C, 14, 15>
<start T3>
<T3, D, 19, 20>
<end ckpt>
<commit T2>
<commit T2>
scan
scan
10
Undo/Redo Logging Recovery Rules
Backwards pass (end of log latest valid checkpoint
start)
Constructing set S of committed transactions
undo actions of transactions not in S
undo pending transactions
follow undo chains for transactions in (checkpoint active
list) – S
Forward pass (latest checkpoint start end of log)
redo actions of S transactions
11
11 trang
17140
Free
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