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Introduction to Database Systems1 Logging and Recovery CC Lecture 2.

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2 Introduction to Database Systems1 Logging and Recovery CC Lecture 2

3 Introduction to Database Systems2 Review: The ACID properties v A v A tomicity: all actions in the Xact happen, or none happen v C v C onsistency: if each Xact is consistent, and the DB starts consistent, it ends up consistent v I v I solation: execution of one Xact is isolated from that of other Xacts v D v D urability: if a Xact commits, its effects persist v The Recovery Manager guarantees Atomicity & Durability

4 Introduction to Database Systems3 Motivation v Atomicity: –Transactions may abort (“Rollback”) v Durability: –What if DBMS stops running (causes?) crash! v Desired behavior after system restarts: –T1, T2 & T3 should be durable –T4 & T5 should be aborted (effects not seen). T1 T2 T3 T4 T5

5 Introduction to Database Systems4 Assumptions v Concurrency control is in effect –Strict 2PL, in particular v Updates are happening “in place” –i.e. data is overwritten on (deleted from) the disk. v A simple scheme to guarantee Atomicity & Durability?

6 Introduction to Database Systems5 Handling the Buffer Pool v Force writes to disk? –poor response time –but provides durability v Steal buffer-pool frames from uncommited Xacts? –if not, poor throughput –if so, how to provide atomicity? Force No Force No Steal Steal Trivial Desired

7 Introduction to Database Systems6 Examples v STEAL (why Atomicity is a problem) –steal frame F: some page P is written to disk –what if the Xact with the lock on P aborts? –must remember the old value of P at steal time! u to support UNDOing the write to P v NO FORCE (why Durability is a problem) –how to guarantee durability without writing? u Can’t be done! –So write as little as possible, in a convenient place, at commit time u to support REDOing actions

8 Introduction to Database Systems7 Basic Idea: Logging v Store REDO and UNDO information in a log –for every update, generate UNDO & REDO info –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 v Log: An ordered list of REDO/UNDO actions –log record contains u –and additional control info (which we’ll see soon)

9 Introduction to Database Systems8 Write-Ahead Logging (WAL) v 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 before commit. v #1 guarantees Atomicity v #2 guarantees Durability v Exactly how is logging (and recovery!) done? –We’ll study the ARIES algorithms

10 Introduction to Database Systems9 WAL & the Log v Each log record has a unique Log Sequence Number (LSN) –LSN’s always increasing v Each data page contains a pageLSN –the LSN of the most recent log record for an update to that page. v System keeps track of flushedLSN –the max LSN flushed so far –log records in memory form the “tail” of the log v WAL sez: before a page is written, –pageLSN  flushedLSN LSNs DB pageLSNs RAM flushedLSN

11 Introduction to Database Systems10 Log Records Possible log record types: v Update v Commit v Abort v End (signifies end of commit or abort) v Compensation Log Records (CLRs) –for UNDO actions prevLSN XID type length pageID offset before-image after-image LogRecordupdate records only

12 Introduction to Database Systems11 Other Log-Related State v Transaction Table –one entry per active Xact –contains XID, status (running/commited/aborted), and lastLSN v Dirty Page Table –one entry per dirty page in buffer pool –contains recLSN -- the LSN of the log record which first caused the page to be dirty

13 Introduction to Database Systems12 The Big Picture prevLSN XID type length pageID offset before-image after-image LogRecords update records only DB pageLSNs Xact Table lastLSN status Dirty Page Table recLSN flushedLSN RAM

14 Introduction to Database Systems13 Normal Execution of an Xact v Strict 2PL v Series of reads & writes, followed by commit or abort –assume that write is atomic on disk v STEAL, NO-FORCE buffer management, with Write-Ahead Logging

15 Introduction to Database Systems14 Simple Transaction Abort v For now, consider an explicit abort of a Xact –no crash involved v 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 u for recovering from crash during UNDO!

16 Introduction to Database Systems15 Abort, cont. v To perform UNDO, must have a lock on data! –No problem! v Before restoring old value of a page, write a CLR to the log –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) v At end of UNDO, write an “end” log record

17 Introduction to Database Systems16 Transaction Commit v Write commit record to log. v All log records up to lastLSN are flushed –guarantees that flushedLSN  lastLSN –note that log flushes are sequential, synchronous writes –many log records per log page v Commit() returns v write end record to log

18 Introduction to Database Systems17 Checkpoints v Periodically, want to get a “snapshot” of the DBMS -- speeds up recovery! v new log records: begin_checkpoint, end_checkpoint. –write a begin_checkpoint record as a new Xact –end_checkpoint record contains the current state of the Xact and Dirty Page tables –after end_checkpoint is flushed, the LSN of the begin_checkpoint record is stored in a special master record v Note: this is a “fuzzy checkpoint”! –no locking involved. good as of begin_checkpt.

19 Introduction to Database Systems18 Recovering from a Crash v Start from a checkpoint (found via master record) v Three phases. Need to: –figure out which Xacts committed since checkpoint, which failed (Analysis) –REDO all actions (repeat history) –UNDO effects of failed Xacts Oldest log rec. of Xact active at crash Smallest recLSN in dirty page table after Analysis Last chkpt CRASH A RU

20 Introduction to Database Systems19 Recovery: The Analysis Phase v reconstruct state at checkpoint –via end_checkpoint record v scan log forward from chkpt. –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 D.P.T. u add P to dirty page table, set recLSN=LSN

21 Introduction to Database Systems20 Recovery: The REDO Phase v Repeat History to reconstruct state at crash: –reapply all updates (even of aborted Xacts!) –redo any actions in CLRs v Start with smallest recLSN in D.P.T. Redo each action unless: –affected page is not in the Dirty Page Table –affected page is in DPT, but has recLSN > LSN –pageLSN (in DB)  LSN v To REDO an action: –reapply logged action –set pageLSN to LSN. No additional logging!

22 Introduction to Database Systems21 Recovery: The UNDO Phase v ToUndo={ l | l a lastLSN of a “loser” Xact} v Repeat: –choose largest LSN among ToUndo –if this LSN is a CLR and undonextLSN==NULL u write an End record for this Xact –if this LSN is a CLR, and undonextLSN != NULL u Add undonextLSN to ToUndo u (Q: what happens to other CLRs?) –else this LSN is an update. Undo the update, write a CLR, add prevLSN to ToUndo. Until ToUndo is empty.

23 Introduction to Database Systems22 Example of Recovery 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 LSN LOG 00 05 10 20 30 40 45 50 60 Xact Table lastLSN status Dirty Page Table recLSN flushedLSN ToUndo prevLSNs RAM

24 Introduction to Database Systems23 Example: Crash During Restart! 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 LSN LOG 00,05 10 20 30 40,45 50 60 70 80,85 90 Xact Table lastLSN status Dirty Page Table recLSN flushedLSN ToUndo undonextLSN RAM

25 Introduction to Database Systems24 Additional Crash Issues v What happens if system crashed during Analysis? During REDO? v How do you limit the amount of work in REDO? –flush asynchronously in the background –watch “hot spots”! v How do you limit the amount of work in UNDO? –avoid long-running Xacts

26 Introduction to Database Systems25 Summary of Logging/Recovery v Recovery Manager guarantees Atomicity & Durability v Use WAL to allow STEAL/NO-FORCE w/o sacrificing correctness v LSNs identify log records; linked into backwards chains per transaction (via prevLSN) v pageLSN allows comparison of data page and log records

27 Introduction to Database Systems26 Summary, Cont. v Checkpointing: a quick way to limit the amount of log to scan on recovery v Recovery works in 3 phases –Analysis since checkpoint –Redo since oldest recLSN –Undo from end to first LSN of oldest Xact alive at crash v Upon Undo, write CLRs v Redo “repeats history”: simplifies the logic!

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