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Failure Recovery

CheckpointingUndo/Redo Logging

Source: slides by Hector Garcia-Molina

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Recovery is very, very

SLOW !Redo log:

First T1 wrote A,B LastRecord Committed a year ago

Record(1 year ago) --> STILL, Need to redo after crash!!

... ... ...

Crash

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Solution: Checkpoint (simple version)

Periodically:(1) Do not accept new transactions(2) Wait until all transactions finish(3) Flush all log records to disk (log)(4) Flush all buffers to disk (DB) (do not discard

buffers)

(5) Write “checkpoint” record on disk (log)

(6) Resume transaction processing

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Example: what to do at recovery?

Redo log (disk):

<T1

,A,1

6>

<T1

,com

mit

>

Ch

eck

poin

t

<T2

,B,1

7>

<T2

,com

mit

>

<T3

,C,2

1>

Crash... ... ... ...

...

...

Start from last checkpoint and move forwardin the log file redoing updates for committedtransactions.

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Key drawbacks:

Undo logging: data must be written to disk immediately after a transaction finishes, which can increase number of disk I/O's

Redo logging: need to keep all modified blocks in memory until transaction commits and log is flushed, which can increase the number of buffers required

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Solution: undo/redo logging!

Update record in the log has the format

<T, X, new X val, old X val>

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Rules

Buffer containing X can be flushed to disk either before or after T commits

Log record must be flushed to disk before corresponding updated buffer is (WAL)

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Recovery with Undo/Redo Logging

1. Redo all committed transactions in order from earliest to latest

handles committed transactions with some changes not yet on disk

2. Undo all incomplete transactions in order from latest to earliest

handles uncommitted transactions with some chnages already on disk

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Non-quiescent Checkpoint

Simple checkpointing scheme requires system to "quiesce" (reach a point with no active transactions), ensured by preventing new transactions from starting for a while

Avoid this behavior with non-quiescent checkpointing: write a "start checkpoint" record to the log later write an "end checkpoint" record to the log

Details vary depending on whether undo, redo, or undo/redo logging

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Non-quiescent Checkpoint for Undo/Redo

write "start checkpoint" listing all active transactions to log

flush log to disk write to disk all dirty buffers (contain a

changed DB element), whether or not transaction has committed this implies some log records may need to be

written to disk (WAL) write "end checkpoint" to log flush log to disk

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Non-quiescent checkpoint for undo/redo

LOG

for undo dirty buffer

pool pagesflushed

start ckptactive T's:T1,T2,...

endckpt

.........

...

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Recovery process: Backwards pass (end of log latest checkpoint start)

construct 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

backward pass

forward passstart

check-point

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Examples what to do at recovery time?

no T1 commit

LOG

T1,-a

...CkptT1

...Ckptend

...T1-b

...

Undo T1 (undo a,b)

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Example

LOG

...T1

a... ...

T1

b... ...

T1

c...

T1

cmt...

ckpt-end

ckpt-sT1

Redo T1: (redo b,c)

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Real world actions

E.g., dispense cash at ATMTi = a1 a2 …... aj …... an

$

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Solution

(1) execute real-world actions after commit

(2) try to make idempotent

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Media failure (loss of non-volatile storage)

A: 16

Solution: Make copies of data!

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Example 1 Triple modular redundancy

Keep 3 copies on separate disks Output(X) --> three outputs Input(X) --> three inputs + vote

X1 X2 X3

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Example 2 Redundant writes, Single reads

Keep N copies on separate disks Output(X) --> N outputs Input(X) --> Input one copy

- if ok, done- else try

another one Assumes bad data can be detected

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Example 3: DB Dump + Log

backupdatabase

activedatabase

log

• If active database is lost,– restore active database from backup– bring up-to-date using redo entries in log

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When can log be discarded?

check-point

dbdump

lastneededundo

not needed formedia recovery

not needed for undoafter system failure

not needed forredo after system failure

log

time

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Summary

Consistency of data One source of problems: failures

- Logging- Redundancy

Another source of problems: Data Sharing..... next