Transaction Log Performance Tuning

1. Transaction Log Performance Tuning Chirag Roy – Senior SQL DBA MCITP: Database Developer 2005/2008 MCITP: Database Administrator 2005/2008 http://sqlking.wordpress.com http://www.twitter.com/chiragroy

2. Agenda • Transaction Log Architecture • Design Options for Performance • Hardware Options for Performance • Transaction Log Troubleshooting • Summary

3. Transaction Log architecture

4. Transaction Log Architecture Physical/Logical Architecture Logical Log File Virtual Log Files * http://msdn.microsoft.com/en-us/library/ms179355.aspx

5. Transaction Log Architecture Lazy Writer Checkpoint Buffer Pool Plan Cache Dirty Page Transaction Log Full Recovery Mode Simple Recovery Mode Data Cache Data file

6. Transaction Log Architecture Recovery Type Considerations - • Simple Recovery - Log file cleared on checkpoint • Full/Bulk Logged Recovery – Log file cleared on Log Backup • Bulk Logged Recovery • Potentially Larger Log Backups when running - • ALTER INDEX REORGANIZE • DBCC INDEXDEFRAG

7. Transaction Log Architecture Tools to Check T-LOG - • DBCC LOGINFO

8. Transaction Log Architecture Tools to Check T-LOG - • DBCC SQLPERF(LOGSPACE)

9. Transaction Log Architecture Tools to Check T-LOG - • Disk Usage Report • TRACE FLAG 3004

10. Design Options for Performance

11. Design options for performance VLF Design • Too few Large VLF’s due to poor design • Too many Small VLF’s in case of Autogrow • Smallest Log File Size can be 512KB on creation • VLF Sizing should be carefully planned according to environment needs

12. Design options for performance VLF Design

13. Design options for performance VLF Design • If log file designed for VLDBs > 8GB, expand Log File in Increments of 8GB on DB Creation to create 512MB VLFs • If log file designed < 8GB, size Log File as per requirements

14. Design options for performance Considerations - • Autoshrink is Evil – Switch OFF • Autogrowth by % is Evil’er, causes VLF Fragmentation • VLF Fragmentation - • Leads to I/O overhead • Affects Redo/Undo phase performance • Increases database recovery/restore time • Cluster Failover Timing

15. Design options for performance Considerations - • Place Data and Log files on separate LUNS to distribute I/O • Data Files experience Random Read/Writes • Log Files experience Sequential Read/Writes • SAN Admins need to provision LUNS optimized for the type of load

16. Design options for performance Considerations - • Change Model Database Recovery Mode to Simple • Full Recovery Database in Pseudo Simple Until First Full Backup • Runaway Log file if subsequently no Log backups are taken • Instant File Initialization does not work with Log Files • When Restoring Database create database first with properly sized data and log files

17. Design options for performance Considerations - • Log clearing can be affected by – • Recovery Model • Replication • Database Mirroring • Switch on Backup Compression in SQL 2008/R2

18. Design options for performance TempDB - Special Case • In Large OLTP Environment Size Tempdb data and log file appropriately • Test using Autogrow • Size before going into production • Checkpoint occurs when Log File is 70% Full • Slow Disk I/O can cause delayed checkpoint • Mitigate using Alerts to notify • Manual Checkpoint precedes over System Checkpoint

19. Hardware Options for Performance

20. Hardware Options RAID 1 • Good Read, Slower Write Performance • Good Redundancy • Data Availability • Expensive *http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/strcnpts.html

21. Hardware Options RAID 10 • Good Read/Write Performance • Good Redundancy • Data Availability • More Expensive *http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/strcnpts.html

22. Hardware Options SSD • Extremely Good Read + Good Write Performance • Good Redundancy • Data Availability • Very Expensive * http://www.fusionio.com/load/media-imagesMediakit/gsyhv/image6_orig.jpg?attach=1

23. Hardware Options Disk Sector Alignment • Still on Windows 2003 make sure to use disk sector alignment • Read Jimmy May’s blogs or whitepaper • http://blogs.msdn.com/jimmymay/archive/tags/Disk+Partition+Alignment/default.aspx • http://msdn.microsoft.com/en-us/library/dd758814.aspx • In Windows 2008, disk sectors are aligned to 1MB by default for disks larger than 4GB

24. Transaction Log troubleshooting

25. hardware utilisation and performance Storage • Check the file latency within SQL Server using • sys.dm_io_virtual_file_stats(db_id,file_id) • Use this script to get the latency for each file: select db_name(database_id), io_stall_read_ms/num_of_reads AS 'Disk Read Transfer/ms', io_stall_write_ms/num_of_writes AS 'Disk Write Transfer/ms' from sys.dm_io_virtual_file_stats (2,1)

26. Dynamic management views sys.dm_os_waiting_tasks • Wait information • Task level • Very accurate • Transient data

27. Dynamic management views sys.dm_os_wait_stats • Wait information • Cumulative by wait type • Persistent data • Transient data

28. Dynamic management views Log_reuse_wait_desc in sys.databases • NOTHING • CHECKPOINT • LOG_BACKUP • ACTIVE_BACKUP_OR_RESTORE • ACTIVE_TRANSACTION • DATABASE_MIRRORING • REPLICATION • DATABASE_SNAPSHOT_CREATION • LOG_SCAN • OTHER_TRANSIENT

29. Common wait types ASYNC_IO_COMPLETION • Can be for "zeroing" out a transaction log file during log creation or growth WRITELOG • Writing transaction log to disk LOGBUFFER • Indicates worker thread is waiting for a log buffer to write log blocks for a transaction *http://blogs.msdn.com/psssql/archive/2009/11/03/the-sql-server-wait-type-repository.aspx

30. Demo

31. summary

32. SUMMARY • VLF Design • Switch of AutoShrink • Use Autogrow as last resort • Enable Compression in SQL 2008/R2 • Log files on Faster Dedicated Disks • significant resource waits • ASYNC_IO_COMPLETION • WRITELOG • LOGBUFFER

33. THANK YOU & Questions