1Logging and RecoveryChapter 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 otherXacts.••DDurability: If a Xact commits, its effects persist.• The Recovery Manager guarantees Atomicity & Durability.Motivation• Atomicity: – Transactions may abort (“Rollback”).• Durability:– What if DBMS stops running? (Causes?)crash!Y Desired Behavior after system restarts:– T1, T2 & T3 should be durable.– T4 & T5 should be aborted (effects not seen).T1T2T3T4T5Assumptions• Concurrency control is in effect. – Strict 2PL, in particular.• Updates are happening “in place”.– i.e. data is overwritten on (deleted from) the disk.• A simple scheme to guarantee Atomicity & Durability?2Handling the Buffer Pool• Force write to disk at commit?– Poor response time.– But provides durability.• Steal buffer-pool frames from uncommited Xacts?– If not, poor throughput.– If so, how can we ensure atomicity?ForceNo ForceNo StealStealTrivialDesiredMore on Steal and Force•STEAL (why enforcing Atomicity is hard)–To steal frame F: Current page in F (say P) is written to disk; some Xact holds lock on P.• What if the Xact with the lock on P aborts?• Must remember the old value of P at steal time (to support UNDOing the write to page P).•NO FORCE(why enforcing Durability is hard)– What if system crashes before a modified page is written to disk?– Write as little as possible, in a convenient place, at commit time,to support REDOing modifications.Basic 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 write all log records for a Xactbeforecommit.• #1 guarantees Atomicity.• #2 guarantees Durability.• Exactly how is logging (and recovery!) done?– We’ll study the ARIES algorithms.3WAL & 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:Beforea page is written,–pageLSN≤ flushedLSNLSNs pageLSNsRAMflushedLSNpageLSNLog recordsflushed to disk“Log tail”in RAMDBLog RecordsPossible log record types:• Update•Commit•Abort•End (signifies end of commit or abort)• Compensation Log Records (CLRs)– for UNDO actions– (and some other tricks!)prevLSNXIDtypelengthpageIDoffsetbefore-imageafter-imageLogRecord fields:updaterecordsonlyOther Log-Related State• Transaction Table:– One entry per active Xact.–Contains XID, status (running/commited/aborted), and lastLSN.• Dirty Page Table:– One entry per dirty page in buffer pool.–ContainsrecLSN -- the LSN of the log record which firstcaused the page to be dirty.Normal Execution of an Xact• Series of reads & writes, followed by commit or abort.– We will assume that page 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.4Checkpointing• 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 currentXact table and dirty page table. This is a `fuzzy checkpoint’:• Other Xacts continue to run; so these tables only known to reflect some mix of state after 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 WhereDBData pageseachwith apageLSNXact TablelastLSNstatusDirty Page TablerecLSNflushedLSNRAMprevLSNXIDtypelengthpageIDoffsetbefore-imageafter-imageLogRecordsLOGmaster recordSimple 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.–GetlastLSN of Xact from Xact table.– Can follow chain of log records backward via theprevLSN field.– Note: before starting UNDO, could write an Abort log record.•Why bother?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 happen)• But they might be Redone when repeating history (=1 UNDO guaranteed)• At end of all UNDOs, write an “end” log record.5Transaction Commit• Write commit record to log.• All log records up to Xact’s lastLSN are flushed.– Guarantees that flushedLSN ≥ lastLSN.– Note that log flushes are sequential, synchronous writes to disk.– Many log records per log page.• Make transaction visible– Commit() returns, locks dropped, etc.• Write end record to log.Crash Recovery: Big PictureY Start from a checkpoint (found via master record).Y Three phases. Need to:– Figure out which Xactscommitted since checkpoint, which failed (Analysis).– REDO all actions.X (repeat history)– UNDO effects of failed Xacts.Oldest log rec. of Xactactive at crashSmallestrecLSN in dirty page table after AnalysisLast chkptCRASHA RURecovery: The Analysis Phase• Reconstruct state at checkpoint.–via end_checkpoint