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Recovery, logging and the transaction log
The document discusses transaction log architecture, log records, checkpoints, transaction log operations, recovery and crash recovery, and recovery models and minimal logging in SQL Server. It covers topics such as how the transaction log file is created and grows, what log records are and where they are stored, how checkpoints work and the different types of checkpoints, how recovery brings databases to a consistent state after failures, and the different recovery models including minimal logging.
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Recovery, logging and the transaction log
- 1. ©2011 Hewlett-Packard DevelopmentCompany, L.P. The information contained herein is subject to change without notice Recovery, Logging and the Transaction Log Miroslav Dimitrov
- 2. Agenda • Transaction LogArchitecture • Log Records • Checkpoints • Transaction Log operations • Recovery and Crash Recovery • Recovery Models and Minimal Logging
- 3. Transaction Log Architecture •Creating and growing the log file • Log is always zero-initialized • TF 3004 and 3605 • VLFs, Log blocks and LSNs • Undocumented DBCC LOGINFO • DBCC SQLPERF (Logspace)
- 4. Log Records • Whatare they? • Where they stayed? • Are they all part of transactions? • TVF fn_dblog (startLSN,endLSN) – TF 2537 • Compensation records (rollback) – only roll forward
- 5. Conundrum If a fullbackup is being taken, then you have transaction updated 100 000 records but it is rolled back why is the diff backup so large? Surely nothing has changed.
- 6. Checkpoints • Why theyexists? • Thresholds 20 ms and 100 ms?! • Log records during checkpoint • Log records are written to disk via three ways: – When we have commit/rollbacked transaction – When a data file page is written to disk – When a log block hits the maximum of 60 kb size and must be flushed to disk
- 7. Checkpoints (cont.) • Types: –Automatic – Indirect (new in SQL 2012) – Internal – Manual • TF 3502 (for which db), 3504 (write latency), 3605 • XE Events - checkpoint_begin and checkpoint_end
- 8. Transaction Log operations •VLF is either active or inactive • DBCC OPENTRAN • VLF is made inactive by log clearing (truncation) • Circular nature of log • log_reuse_wait_desc in sys.databases – why log clearing did not succeed last time it was attempted
- 9. Recovery and CrashRecovery • Recovery brings db to a consistent point in time • Types: – Crash recovery after a system failure – Recovery at the end of restore – Recovery after a failover (CL, AG, Mirroring) • REDO • UNDO
- 10. Recovery and CrashRecovery (cont.) • Recovery during restore • Crash Recovery • Where the crash recovery stops?! • How SQL is recovering DBs? – TF 3408 single thread • Fast Recovery (from SQL 2005 onwards – only ENT) – does not apply for restore
- 11. Recovery Models andMinimal Logging • Minimal Logged vs Efficiently Logged • FULL • SIMPLE • BULK-LOGGED (why to use it only temp) • Minimally Logged: – Creating, Dropping, Rebuilding indexes (reorganize is always fully logged) – Bulk operations – bcp, bulk insert etc.
- 12. Recovery Models andMinimal Logging (cont.) • pseudoSimple recovery model? • Is switching between FULL and BULK-LOGGED breaking the log backup chain? • Deferred drop and TRUNCATE table (two ways)
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