Motivation Logging and Recovery Chapter 18 If 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 County Review The ACID properties A tomicity All actions in the Xact happen or none happen C onsistency If each Xact is consistent and the DB starts consistent it ends up consistent I solation 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 Atomicity Transactions may abort Rollback Durability What if DBMS stops running Causes Y Desired Behavior after system restarts T1 T2 T3 should be durable T4 T5 should be aborted effects not seen T1 T2 T3 T4 T5 crash Assumptions 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 1 Handling the Buffer Pool Force write to disk at commit No Steal Poor response time Force Trivial But provides durability Steal buffer pool frames from uncommited Xacts If not poor throughput No Force If so how can we ensure atomicity Basic Idea Logging Steal Desired More on Steal and Force 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 STEAL 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 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 NO FORCE 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 before the corresponding data page gets to disk d Must write all log records for a Xact before commit 1 guarantees Atomicity 2 guarantees Durability Exactly how is logging and recovery done We ll study the ARIES algorithms 2 DB WAL the Log LSNs pageLSNs RAM flushedLSN Each log record has a unique Log Sequence Number LSN Log records flushed to disk LSNs always increasing Each data page contains 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 pageLSN Log tail WAL Before a page is written in RAM pageLSN flushedLSN Log Records LogRecord fields update records only prevLSN XID type pageID length offset before image after image Other 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 Contains recLSN the LSN of the log record which first caused the page to be dirty Normal Execution of an Xact Possible log record types Update Commit Abort End signifies end of commit or abort Compensation Log Records CLRs for UNDO actions and some other tricks 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 Ahead Logging buffer management with Write 3 Checkpointing Simple Transaction Abort 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 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 Note before starting UNDO could write an Abort log record 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 Why bother The Big Picture What s Stored Where LOG DB LogRecords prevLSN XID type pageID length offset before image after image Data pages each with a pageLSN master record RAM Xact Table lastLSN status 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 Dirty Page Table CLR contains REDO info CLRs never Undone recLSN flushedLSN 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 4 Transaction Commit Recovery The Analysis Phase 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 Reconstruct state at checkpoint via end checkpoint record Scan log forward from begin 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 Crash Recovery Big Picture Oldest log rec of Xact active at crash Y Smallest recLSN in dirty page table after Analysis Y Start from a checkpoint found via master record Three phases Need to Figure out which Xacts committed since checkpoint which failed Analysis REDO all actions X repeat history UNDO effects of failed Xacts Last chkpt CRASH A R U Recovery The REDO Phase We repeat History to reconstruct state at crash Reapply all updates 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