ARIES. Handout #24. Overview

Transcription

1 Handout #24 ARIES Overview Failure Model Goals CLRs Data Structures Normal Processing Checkpoints and Restart Media Recovery Nested Top Actions Design Decisions CS346 - Transaction Processing Markus Breunig - ARIES / 1 -

2 General Properties implemented e.g. in OS/2 Ext. Ed. DB Manager log based LSN = logical address of log record UNDO, REDO and UNDO/REDO logging supported physical and logical (operations) logging WAL in-place updating (no shadows) CS346 - Transaction Processing Markus Breunig - ARIES / 2 -

3 Failure Model Three types of failures considered transaction/process system failures system failures media/device failures transactions can be in one of three states forward processing partial rollback total rollback rollbacks can be initiated by system transaction CS346 - Transaction Processing Markus Breunig - ARIES / 3 -

4 Logging page oriented REDO logging independent recovery logical UNDO logging higher concurrency back-linking with PrevLSN field compensation log records (CLRs) written, pure REDO standard buffer management (no steal, fix) with WAL CS346 - Transaction Processing Markus Breunig - ARIES / 4 -

5 Goals simplicity operation logging flexible storage management partial rollbacks flexible buffer management recovery independence logical undo parallelism and fast recovery minimal overhead [see ARIES paper, pages 248ff] CS346 - Transaction Processing Markus Breunig - ARIES / 5 -

6 CLR s pure REDO log records bounded number of records written during rollback/abort/restart UndoNextLSN to skip undone records repeat history logical logging of CLRs CS346 - Transaction Processing Markus Breunig - ARIES / 6 -

7 Checkpoiting & Restart checkpoint contains: Active TRIDs state of active transactions LSNs of most recent log records dirty buffer page array 3 passes during recovery Analysis Pass: from latest checkpoint to end, gather data and reconstruct state REDO Pass: from oldest dirty page LSN to end repeat history no log records written locks reacquired UNDO Pass: backwards from end undo all non-clrs follow back pointers CS346 - Transaction Processing Markus Breunig - ARIES / 7 -

8 Data Structures log records: LSN type TransID PrevLSN UndoNxtLSN Data page structure: page_lsn data transaction table: TransID state LastLSN UndoNxtLSN dirty page buffer table: PageID recoverylsn CS346 - Transaction Processing Markus Breunig - ARIES / 8 -

9 Normal Processing Transaction can be in one of the following states: forward processing partial rollback total rollback Rollback can be application initiated (abort_work) system initiated (deadlock, crash) bounded undo-logging amount allows circular logs termination of transactions write prepare record, contains write locks (IX, X, SIX, ) pending actions CS346 - Transaction Processing Markus Breunig - ARIES / 9 -

10 Checkpoints taken periodically fuzzy master record log anchor end_chkpt record contains transaction table dirty page table file mappings spread buffer writes over entire checkpoint period no log records forced CS346 - Transaction Processing Markus Breunig - ARIES / 10 -

11 Restart Processing in Detail Analysis Pass read master record and find latest checkpoint LSN reconstruct transaction table, compute RedoLSN writes only missing transaction_end log records could be suppressed by doing work in Redo Pass Redo Pass no log records written bounded number of pages accessed by estimating if they might have been modified (RecLSN=nextLSN at time non-dirty page was last fixed) parallel execution possible Undo Pass all aborting transaction undone in one log sweep (by taking maximum) writes CLRs, does WAL parallel execution possible Checkpoint persistent savepoints possible CS346 - Transaction Processing Markus Breunig - ARIES / 11 -

12 Checkpointing During Restart after analysis pass transaction table ok dirty pages table ok during Redo Pass need to redo less log during Undo Pass just like during normal processing CS346 - Transaction Processing Markus Breunig - ARIES / 12 -

13 Media Recovery fuzzy dump - directly from non-volatile storage exploit drive geometry eliminate buffer manager overhead TP Monitor can be down remember most recent checkpoint at time when fuzzy dump started = image copy checkpoint calculate media recovery redo point (mrrp) Recovery: read archive copy, redo from mrrp, undo all uncommitted transactions individual page recovery possible broken page: read stable copy and redo CS346 - Transaction Processing Markus Breunig - ARIES / 13 -

14 Nested Top Actions compare to transaction models we have seen! implementation: ascertain position of transactions last log record log undo/redo for actions of the nested top action on completion of the nested top action: write dummy CLR with UndoNxtLSN pointing to log record remembered in first step if nested top action only one action: optimization possible: log as redo-only log record, write no dummy CLR nested top action durable even if transaction itself aborts! CS346 - Transaction Processing Markus Breunig - ARIES / 14 -

15 Why: no selective REDO instead: repeat history otherwise: no record level locking possible, only page locking page LSN no indicator of current state any more Why: write CLRs otherwise: no repeating of history possible redo logging cannot be physical (efficiency) Why: not have multiple LSNs per page useful to keep track of LSNs for each object in page but overhead too high CS346 - Transaction Processing Markus Breunig - ARIES / 15 -

16 Summary: Attributes of ARIES fine granularity concurrency control and locking flexible buffer management low space overhead no constraints on data to guarantee idempotence of redo/undo operations logging redo-only and undo-only log records partial and total rollback nested top actions checkpoints bounded logging during restart (even if multiple failures) fast restart due to parallelism/deferred processing fuzzy dump persistent checkpoints possible few log passes during recovery CLRs redo only CS346 - Transaction Processing Markus Breunig - ARIES / 16 -

17 Summary one particular algorithm lots of tricks for performance reviewed whole course! CS346 - Transaction Processing Markus Breunig - ARIES / 17 -