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16…. Recovery - Review (only for DB with updates…..!). ACID: Atomicity & Durability Trading execution time for recovery time Mechanics of recovery What is recovered and how Write ahead logging Mechanics of logging Big Picture. Learning Objectives.

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16 recovery review only for db with updates
16…. Recovery - Review (only for DB with updates…..!)
  • ACID: Atomicity & Durability
  • Trading execution time for recovery time
  • Mechanics of recovery
    • What is recovered and how
    • Write ahead logging
    • Mechanics of logging
    • Big Picture
learning objectives
Learning Objectives
  • Describe Force and No Steal policies and their implications
  • Describe write ahead log, commit and abort
  • What is the role of each element in crash recovery
    • LSN, before image, after image, FlushedLSN, pageLSN, dirty page table, transaction table, checkpoint
  • Be able to carry out the three phases of crash recovery in a simple example.
review the acid properties
Review: The ACID properties

Recovery

System

  • Atomicity: All actions in the transaction happen in their entirety or none of them happen.
  • Consistency: If each transaction is consistent, and the DB starts in a consistent state, it ends in a consistent state.
  • Isolation: Execution of one transaction is isolated from that of other transactions.
  • Durability: If a transaction commits, its effects persist.

Programmers

Concurrency

Control

System

Recovery

System

simple solutions to crash recovery
Simple solutions to Crash Recovery
  • The crash recovery subsystem has two problems to solve, atomicity and durability. Here are some really simple ways to handle them.
  • Atomicity
    • Example:
      • Deposit $100 into A, withdraw $100 from B.
      • The OS crashes after $100 is deposited to A.
      • The recovery manager must undo the deposit.
    • Solution (NO STEAL): Make sure no updates (e.g. the deposit) are written to disk until the transaction commits. The transaction will disappear when the OS crashes.
  • Durability
    • Same example, now the OS crashes after the commit and before the withdrawal was written to disk.
    • Solution (FORCE): Write every update to disk just before the transaction commits.
why these simple solutions don t work
Why these simple solutions don’t work
  • No Steal: keep updates in memory until commit
    • If DBMS doesn’t steal, memory is full of updates and other transactions are starved for memory. Poor throughput.
  • Force: write all updates to disk immediately on commit
    • Force keeps the disk too busy and results in poor response time. The records being updated may be hot spots.
    • Why do you have to warn your OS before removing an external device? Because of the OS’s No Force Policy.

No Steal

Steal

Slow

Execution;

Easy recovery

Force

Fast Execution; Tough Recovery

No Force

the solution
The solution
  • Given that we are faced with the No Force and Steal policies, the crash recovery subsystem of every DBMS uses a Write Ahead Log (WAL) to manage crash recovery (and aborts also).
  • The WAL is put on a separate disk from the data (why?). It begins from each backup, which is typically taken each day.
  • A log record is written for every insert, update, delete, begin-trans, commit, abort and checkpoint.
  • A log record contains

<XID, ID of the DB record, action, old data, new data>

before

image

after

image

write ahead log wal
Write Ahead Log (WAL)

To be a write ahead log, the log must obey these rules:

The atomicity rule: the log entry for an insert, update or delete must be written to disk before the changeis made to the DB

The durability rule: All log entries for a transaction must be written to disk before the commit record is written to disk.

practice with a log

T1,A,update,100,200

T2,C,update,1000,500

T2,D,update,500,1000

T2,commit

CRASH

Practice with a log
  • What did each transaction do before the crash?
  • After a crash, what should the recovery manager do to ensure that each transaction is atomic?
    • What guarantees that your solution (UNDO) will work?
  • After a crash, what should the recovery manager do to ensure that each transaction is durable?
    • What guarantees that your solution (REDO) will work?
practice with a log9

T1,A,update,’abc’,’de’

T1,A,update,’de’,’ab’

T2,D,update,10,0

T2,commit

CRASH

Practice with a log
  • What must a recovery manager do after a crash to ensure the atomic and durability properties of ACID?
  • What are the final values of A and D?
  • Does the recovery manager return the DB to its state at the time of the crash?
real recovery is more complicated
Real recovery is more complicated
  • We have ignored many complexities in crash recovery
    • Managing normal aborts, some of which may be in progress at the time of the crash
    • Managing inserts and deletes
    • Supporting multiple lock levels
    • Managing updates to structures like B+ trees when pages split
    • Handling crashes that happen in the middle of recovery
  • In the early days of DBMSs many inflexible, inefficient and even incorrect recovery algorithms were implemented.
aries
ARIES
  • In the early 1990s, C. Mohan of IBM proposed a relatively simple recovery algorithm called ARIES*
  • ARIES differs from our simple model in a few ways
    • It redoes every update, not just those of committed transactions. This simplifies the algorithm.
    • It logs changes when normal aborts are undone. This handles recovery for normal aborts.
    • It logs undos during recovery. This handles crashes during recovery.
    • And….

* Algorithms for Recovery and Isolation Exploiting Semantics

aries runs in phases
ARIES Runs in Phases

time

Three phases.

  • Figure out which transactions are incomplete (ANALYSIS).
  • REDO actions for all transactions.
  • UNDO all actions of incomplete transactions.

Start of log

CRASH

A

R

U

aries uses checkpoints
ARIES Uses Checkpoints
  • Typically a backup is taken each night and the log begins anew each morning. If there is a crash in the afternoon, it may take hours to recover. This is unacceptable.
  • So the DBMS periodically (like every few minutes) takes a checkpoint.
  • At a checkpoint* the DBMS writes all dirty pages to disk and writes to the log a checkpoint record containing the IDs of all active transactions.
  • Then the recovery algorithm can begin at the last checkpoint and recovery is much faster.

*This is not the text/ARIES definition of checkpoint but it is what most real DBMSs use

aries phases with checkpoints
ARIES Phases with Checkpoints

Start from a checkpoint.

Figure out which transactions are incomplete (ANALYSIS).

REDO all actions since the checkpoint.

UNDO all actions of incomplete transactions.

time

Last chkpt

CRASH

A

R

U

practice with a log15

CHECKPOINT T3,T4 active T1,A,update,’ABC’,’DEF’

T3,X,update,’ABC’,DEF’

T2,C,update,1000,500

T4,Y,update,4.1,5.2

T2,D,update,500,1000

T4,commit

T1,A,update,’DEF’,’GHI’

T2,commit

CRASH

Practice with a log
  • What does ARIES do?
  • Why doesn’t ARIES REDO T4’s actions before the checkpoint?
  • Why does UNDO process log entries before the checkpoint?
using the wal to manage aborts
Using the WAL to manage aborts
  • We have seen that a Write Ahead Log, with ARIES, makes atomicity and durability easy to achieve.
  • A Write Ahead Log also makes transaction abort simple. A transaction does not have to keep track of the changes it has made to the DB so it can undo them in case of abort. It just looks at the Write Ahead Log!
aborting a transaction
Aborting a transaction

T1,A,update,’ABC’,’DEF’

T2,C,update,1000,500

T2,D,update,500,1000

T1,B,update,300,400

T1,A,update,’DEF’,’GHI’

T1,abort

  • Note that this is normal processing – no crash in sight
  • What actions must the DBMS take to abort T1?
  • In what order should these actions be taken?
  • What guarantees that all of T1’s changes to the DB have been undone?
  • What if the update to B was not written to disk?
chapter 18 crash recovery
Chapter 18: Crash Recovery
  • Write-Ahead Log
  • ARIES Algorithm
    • Parameters
      • LSN, pageLSN, FlushedLSN
    • Log Record Details
      • PrevLSN, Before_image, After_image, CLR
    • Transaction Table, Dirty Page Table
    • Checkpointing
  • Examples
    • Analysis, Redo, Undo
crash recovery review the acid properties

18. Crash

Crash RecoveryReview: The ACID properties
  • Atomicity: All actions in the Xact happen, or none happen.
  • Consistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent.
  • Isolation: Execution of one Xact is isolated from that of other Xacts.
  • D urability: If a Xact commits, its effects persist.
  • The Recovery Manager guarantees Atomicity & Durability.
basic idea logging

18. Crash

Basic Idea: Logging
  • We’ll study the ARIES algorithms.
      • Algorithm for Recovery and Isolation Exploiting Semantics
  • Record REDO and UNDO information, for every update, in a log.
    • Sequential writes to log (put it on a separate disk).
    • Minimal info (diff) written to log, so multiple updates fit in a single log page.
  • Log: An ordered list of REDO/UNDO actions
    • Log record contains:

<LSN, XID, pageID, offset, length, old data, new data>

    • and additional control info (which we’ll see soon).
example log
Example Log

LSN

  • update: T1 writes P5
  • update: T1 writes P3
  • 30 T1 commit
  • 40 T1 end
  • 50 update: T3 writes P1
  • 60 update: T3 writes P3
  • CRASH, RESTART

write ahead logging wal

18. Crash

Write-Ahead Logging (WAL)
  • The Write-Ahead Logging Protocol:
    • Must force the log record for an update before the corresponding data page gets to disk.
    • Must write all log records for a Xact beforecommit.
  • #1 guarantees Atomicity.
  • #2 guarantees Durability.
  • Exactly how is WAL managed? Key idea is PageLSN
motivation pagelsn
Motivation: PageLSN
  • At any time, memory contains database, log pages
    • Changes to database records are reflected in log records
  • Suppose a database page is to be written to disk
    • Must find all in-memory log records for changes to records on that database page
    • Must write their log pagessequentially, before the database page
  • How to keep track of those log records?
    • Keep the last, largest, of the log records for that page
    • Called PageLSN
    • Stored with that page, in memory and in database
  • Important conclusions:
    • If Log entry# PageLSN is on disk, it’s safe to write the database page
    • A log entry, with LSN <= the PageLSN on disk, has been written to disk.
wal the log

18. Crash

DB

RAM

LSNs

pageLSNs

flushedLSN

pageLSN

WAL & the Log
  • Each log record has a unique Log Sequence Number (LSN).
    • LSNs always increasing.
  • Each data pagecontains a pageLSN.
    • The LSN of the most recent log record for an update to that page.
  • System keeps track of flushedLSN.
    • The max LSN flushed so far.
  • WAL:Before a page is written,
    • pageLSN £ flushedLSN

Log records

flushed to disk

“Log tail”

in RAM

log records

18. Crash

Log Records

LogRecord fields:

Possible log record types:

  • Update
  • Commit
  • Abort
  • End (signifies end of commit or abort)
  • Compensation Log Records (CLRs)
    • for UNDO actions

LSN

prevLSN

XID

type

pageID

length

update

records

only

offset

before-image

after-image

compensation log records
Compensation Log Records
  • A CLR is written whenever an update is undone
  • A CLR represents a change to the database
  • CLRs are redone but not undone
  • Each CLR contains a field undoNextLSN
    • The LSN of the next log record to be undone
key aries ideas
Key ARIES Ideas
  • Write Ahead Logging
    • We’ve seen how that is managed with PageLSN
    • Enables atomicity and durability with the other two ideas
  • REDO of committed Xacts
    • Why is it needed?
    • What does it involve?
  • UNDO of uncommitted Xacts
    • Why is it needed?
    • What does it involve?
motivation reclsn
Motivation: recLSN
  • A page in memory contains many records
    • Some records may be dirty, making the page dirty.
  • During recovery, need to redo dirty records in dirty pages
  • Problem: where in log to start redoing?
  • Answer: earliest log record of all dirty records
    • Called recLSN
    • Stored in dirty page table
  • Important conclusion:
    • All updates before recLSN are clean
other log related state in memory

18. Crash

Other Log-Related State in Memory
  • Dirty Page Table:
    • One entry per dirty page in buffer pool.
    • Contains recLSN -- the LSN of the log record which firstcaused the page to be dirty.
  • Transaction Table:
    • One entry per active Xact.
    • Contains XID, status (running/commited/aborted), and lastLSN.
normal execution of an xact

18. Crash

Normal Execution of an Xact
  • Series of reads & writes, followed by commit or abort.
    • We will assume that write is atomic on disk.
      • In practice, additional details to deal with non-atomic writes.
  • Strict 2PL.
  • STEAL, NO-FORCE buffer management, with Write-Ahead Logging.
checkpointing

18. Crash

Checkpointing
  • Periodically, the DBMS creates a checkpoint, in order to minimize the time taken to recover in the event of a system crash. Write to log:
    • begin_checkpoint record: Indicates when chkpt began.
    • end_checkpoint record: Contains current Xact table and dirty page table. This is a `fuzzy checkpoint’:
      • Other Xacts continue to run; so these tables are accurate only as of the time of the begin_checkpoint record.
      • No attempt to force dirty pages to disk; effectiveness of checkpoint limited by oldest unwritten change to a dirty page. (So it’s a good idea to periodically flush dirty pages to disk!)
    • Store LSN of chkpt record in a safe place (master record).
the big picture what s stored where

18. Crash

The Big Picture: What’s Stored Where

LOG

RAM

DB

LogRecords

Xact Table

lastLSN

status

Dirty Page Table

recLSN

flushedLSN

LSN

Data pages

each

with a

pageLSN

prevLSN

XID

type

pageID

length

master record

offset

before-image

after-image

example log33

PgID recLSN

P5

P6

P7

XID lastLSN

T1

T2

Example: Log

LOG

Undo nextLSN

LSN

10

20

30

40

50

prevLSN XID Type PgID length offset before after

T1 update P5 3 21 ABC DEF

T2 update P6 3 41 HIJ KLM

T2 update P5 3 10 GDE QRS

T1 update P7 3 21 TUV WXY

T1 abort

DIRTY PAGE

TABLE

TRANSACTION

TABLE

simple transaction abort

18. Crash

Simple Transaction Abort
  • For now, consider an explicit abort of a Xact.
    • No crash involved.
  • We want to “play back” the log in reverse order, UNDOing updates.
    • Get lastLSN of Xact from Xact table.
    • Can follow chain of log records backward via the prevLSN field.
    • Before starting UNDO, write an Abort log record.
      • For recovering from crash during UNDO!
abort cont

18. Crash

Abort, cont.
  • To perform UNDO, must have a lock on data.
    • No problem!
  • Before restoring old value of a page, write a CLR:
    • You continue logging while you UNDO!!
    • CLR has one extra field: undonextLSN
      • Points to the next LSN to undo (i.e. the prevLSN of the record we’re currently undoing).
    • CLRs never Undone (but they might be Redone when repeating history: guarantees Atomicity!)
  • At end of UNDO, write an “end” log record.
transaction commit

18. Crash

Transaction Commit
  • Write commit record to log.
  • Release all locks
  • All log records up to Xact’s lastLSN are flushed.
    • Guarantees that flushedLSN ³ lastLSN.
    • Note that log flushes are sequential, synchronous writes to disk.
    • Many log records per log page.
  • Commit() returns.
  • Write end record to log.
crash recovery big picture

18. Crash

Crash Recovery: Big Picture

Oldest log rec. of Xact active at crash

  • Start from a checkpoint (found via master record).
  • Three phases. Need to:
    • Figure out which Xacts committed since checkpoint, which failed (Analysis).
    • REDOall actions.
      • (repeat history)
    • UNDO effects of failed Xacts.

Smallest recLSN in dirty page table after Analysis

Last chkpt

CRASH

A

R

U

recovery the analysis phase

18. Crash

Recovery: The Analysis Phase
  • Reconstruct state at checkpoint.
    • via end_checkpoint record.
  • Scan log forward from checkpoint.
    • End record: Remove Xact from Xact table.
    • Other records: Add Xact to Xact table, set lastLSN=LSN, change Xact status on commit.
    • Update record: If P not in Dirty Page Table,
      • Add P to D.P.T., set its recLSN=LSN.
example log39

PgID recLSN

XID lastLSN

Example: Log

LOG

Undo nextLSN

LSN

10

20

30

40

50

60

prevLSN XID Type PgID length offset before after

T1 update P5 3 21 ABC DEF

T2 update P6 3 41 HIJ KLM

T2 update P5 3 10 GDE QRS

T1 update P7 3 21 TUV WXY

T2 commit/end

T1 update P3 3 14 ABC DEF

CRASH

DIRTY PAGE

TABLE

  • Assume P6 (only) written to disk. What is its PageLSN?
  • Assume LSN60 (only) not written to disk. What if P3 was written to disk?

TRANSACTION

TABLE

recovery the redo phase

18. Crash

Recovery: The REDO Phase
  • We repeat History to reconstruct state at crash:
    • Reapply allupdates (even of aborted Xacts!), redo CLRs.
  • Scan forward from log rec containing smallest recLSN in D.P.T. For each CLR or update log rec LSN, REDO the action unless:
    • Affected page is not in the Dirty Page Table, or
    • Affected page is in D.P.T., but has recLSN > LSN, or
    • pageLSN (in DB) ³ LSN.
  • To REDO an action:
    • Reapply logged action.
    • Set pageLSN to LSN. No additional logging!
example log41

PgID recLSN

P5 10

P6 20

P7 40

XID lastLSN

T1 40

TRANSACTION

TABLE

Example: Log

LOG

Undo nextLSN

LSN

10

20

30

40

50

prevLSN XID Type PgID length offset before after

T1 update P5 3 21 ABC DEF

T2 update P6 3 41 HIJ KLM

T2 update P5 3 10 GDE QRS

T1 update P7 3 21 TUV WXY

T2 commit/end

DIRTY PAGE

TABLE

  • Assume P6 (only) written to disk.
recovery the undo phase

18. Crash

Recovery: The UNDO Phase

ToUndo={ l | l a lastLSN of a “loser” Xact}

Repeat:

  • Choose largest LSN among ToUndo.
  • If this LSN is a CLR and undonextLSN==NULL
    • Write an End record for this Xact.
  • If this LSN is a CLR, and undonextLSN != NULL
    • Add undonextLSN to ToUndo
  • Else this LSN is an update. Undo the update, write a CLR, add prevLSN to ToUndo.

Until ToUndo is empty.

example

18. Crash

LSN LOG

Example

00

05

10

20

30

40

45

50

60

begin_checkpoint

end_checkpoint

update: T1 writes P5

update T2 writes P3

T1 abort

CLR: Undo T1 LSN 10

T1 End

update: T3 writes P1

update: T2 writes P5

PgID recLSN

prevLSNs

DIRTY PAGE

TABLE

XID lastLSN

TRANSACTION

TABLE

example crash during restart

18. Crash

RAM

Example: Crash During Restart!

LSN LOG

00,05

10

20

30

40,45

50

60

70

80,85

90

begin_checkpoint, end_checkpoint

update: T1 writes P5

update T2 writes P3

T1 abort

CLR: Undo T1 LSN 10, T1 End

update: T3 writes P1

update: T2 writes P5

CRASH, RESTART

CLR: Undo T2 LSN 60

CLR: Undo T3 LSN 50, T3 end

CRASH, RESTART

CLR: Undo T2 LSN 20, T2 end

undonextLSN

Xact Table

lastLSN

status

Dirty Page Table

recLSN

flushedLSN

ToUndo

additional crash issues

18. Crash

Additional Crash Issues
  • What happens if system crashes during Analysis? During REDO?
  • How do you limit the amount of work in REDO?
    • Flush asynchronously in the background.
    • Watch “hot spots”!
  • How do you limit the amount of work in UNDO?
    • Avoid long-running Xacts.
summary of logging recovery

18. Crash

Summary of Logging/Recovery
  • Recovery Manager guarantees Atomicity & Durability.
  • Use WAL to allow STEAL/NO-FORCE w/o sacrificing correctness.
  • LSNs identify log records; linked into backwards chains per transaction (via prevLSN).
  • pageLSN allows comparison of data page and log records.
summary cont

18. Crash

Summary, Cont.
  • Checkpointing: A quick way to limit the amount of log to scan on recovery.
  • Recovery works in 3 phases:
    • Analysis: Forward from checkpoint.
    • Redo: Forward from oldest recLSN.
    • Undo: Backward from end to first LSN of oldest Xact alive at crash.
  • Upon Undo, write CLRs.
  • Redo “repeats history”: Simplifies the logic!
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