Crash RecoveryCS 186 Fall 2002, Lecture 25R&G - Chapter 18If you are going to be in the logging business, one of the things that you have to do is to learn about heavy equipment.Robert VanNatta, Logging History of Columbia CountyReview: The ACID properties••AAtomicity: All actions in the Xact happen, or none happen.••CConsistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent.••IIsolation: Execution of one Xact is isolated from that of other Xacts.••DDurability: If a Xact commits, its effects persist.• Question: which ones does the Recovery Managerhelp with?Atomicity & Durability (andalso used for Consistency-relatedrollbacks)Motivation• Atomicity: – Transactions may abort (“Rollback”).• Durability:– What if DBMS stops running? (Causes?)crash!Y Desired state after system restarts:– T1 & T3 should be durable.– T2, T4 & T5 should be aborted (effects not seen).T1T2T3T4T5AbortCommitCommitAssumptions• Concurrency control is in effect. – Strict 2PL, in particular.• Updates are happening “in place”.– i.e. data is overwritten on (deleted from) the actual page copies (not private copies).• Can you think of a simplescheme (requiring no logging) to guarantee Atomicity & Durability?– What happens during normal execution?– What happens when a transaction commits?– What happens when a transaction aborts?Buffer Mgmt Plays a Key Role• Force policy – make sure that every update is on disk before commit.– Provides durability without REDO logging.– But, can cause poor performance.• No Steal policy – don’t allow buffer-pool frames with uncommitedupdates to overwrite committed data on disk.– Useful for ensuring atomicity without UNDO logging.– But can cause poor performance.Of course, there are some nasty details for getting Force/NoSteal to work…Preferred Policy: Steal/No-Force• This combination is most complicated but allows for highest performance.•NO FORCE(complicates enforcing Durability)– What if system crashes before a modified page written by a committed transaction makes it to disk?– Write as little as possible, in a convenient place, at commit time, to support REDOing modifications.•STEAL(complicates enforcing Atomicity)– What if the Xact that performed udpates aborts?– What if system crashes before Xact is finished?– Must remember the old value of P (to support UNDOingthe write to page P).Buffer Management summaryForceNo ForceNo Steal StealNo REDONo UNDOUNDONo REDOUNDOREDONo UNDOREDOForceNo ForceNo Steal StealSlowestFastestPerformanceImplicationsLogging/RecoveryImplicationsBasic Idea: Logging• 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: <XID, pageID, offset, length, old data, new data> – and additional control info (which we’ll see soon).Write-Ahead Logging (WAL)• The Write-Ahead Logging Protocol:c Must force the log record for an update beforethe corresponding data page gets to disk.d Must force all log records for a Xactbefore commit. (alt. transaction is not committed until all of its log records including its “commit” record are on the stable log.)• #1 (with UNDO info) helps guarantee Atomicity.• #2 (with REDO info) helps guarantee Durability.• This allows us to implement Steal/No-Force• Exactly how is logging (and recovery!) done?– We’ll look at the ARIES algorithms from IBM.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 page i is written to DBlog must satisfy:pageLSNi≤ flushedLSNLSNs pageLSNsRAMflushedLSNpageLSNLog recordsflushed to disk“Log tail”in RAMflushedLSNDBLog RecordsprevLSN is the LSN of the previous log record written by thisXact (so records of an Xact form a linked list backwards in time)Possible log record types:• Update, Commit, Abort•Checkpoint (for log maintainence)• Compensation Log Records (CLRs)–for UNDO actions•End (end of commit or abort)LSNprevLSNXIDtypelengthpageIDoffsetbefore-imageafter-imageLogRecord fields:updaterecordsonlyOther Log-Related State• Two in-memory tables:• Transaction Table– One entry per currently active Xact.• entry removed when Xact commits or aborts– Contains XID, status (running/committing/aborting), and lastLSN (most recent LSN written by Xact).• Dirty Page Table:– One entry per dirty page currently in buffer pool.– Contains recLSN -- the LSN of the log record which firstcaused the page to be dirty.The Big Picture: What’s Stored WhereDBData pageseachwith apageLSNXact TablelastLSNstatusDirty Page TablerecLSNflushedLSNRAMLSNprevLSNXIDtypelengthpageIDoffsetbefore-imageafter-imageLogRecordsLOGMaster recordNormal Execution of an Xact• Series of reads & writes, followed by commit or abort.– We will assume that disk write is atomic.• In practice, additional details to deal with non-atomic writes.• Strict 2PL. • STEAL, NO-FORCE buffer management, with Write-Ahead Logging.Transaction Commit• Write commit record to log.• All log records up to Xact’s commit record are flushed to disk.– Guarantees that flushedLSN ≥ lastLSN.– Note that log flushes are sequential, synchronous writes to disk.– Many log records per log page.• Commit() returns.• Write end record to log.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.–Write an Abort log record before starting to rollback operations– Can follow chain of log records backward via the prevLSNfield.– Write a “CLR” (compensation log record) for each undone operation.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).– CLR contains REDO info–CLRsneverUndone • Undo needn’t be idempotent (>1 UNDO won’t
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