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Motivation for Recovery

Motivation for Recovery. Atomicity: Transactions may abort (“Rollback”). Durability: What if DBMS stops running? (Causes?). Desired Behavior after system restarts: T1, T2 & T3 should be durable. T4 & T5 should be aborted (effects not seen). crash!. T1 T2 T3 T4 T5.

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Motivation for Recovery

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  1. Motivation for Recovery • Atomicity: • Transactions may abort (“Rollback”). • Durability: • What if DBMS stops running? (Causes?) • Desired Behavior after system restarts: • T1, T2 & T3 should be durable. • T4 & T5should be aborted (effects not seen). crash! T1 T2 T3 T4 T5

  2. Handling the Buffer Pool • Force every write to disk? • Poor response time. • But provides durability. • Steal buffer-pool frames from uncommitted transactions? • If not, poor throughput. • If so, how can we ensure atomicity? No Steal Steal Force Trivial Desired No Force

  3. 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 (difference) 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 information. • 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 transaction beforecommit.

  4. 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

  5. Recovery • Three steps: (a la` ARIES) • Starting from a checkpoint: • Analysis: figure out which transactions committed • since the checkpoint, and which failed. • REDOall actions in the log. • UNDO effects of failed transactions.

  6. Summary • Users access the database concurrently, and sometimes there are • crashes. • Transactions are sets of operations that are guaranteed to be atomic. • The DBMS guarantees: Atomicity, Consistency, Isolation, • Durability. • Isolation and consistency are guaranteed via locking: 2-phase • (need special care for deadlocks). • Atomicity and durability are guaranteed by: • Logging • Recovery manager (that uses the log). There are MANY MANY more missing details!

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