READPAST & Furious - Transactions, Locking and Isolation. PASS Summit 2012 Edition
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The document outlines a presentation by Mark Broadbent on database transactions, locking mechanisms, and isolation levels, emphasizing their complexities and interdependencies. It covers various transaction types, locking structures, and the implications of isolation levels in SQL Server, including practical examples and scenarios. The presentation also discusses concurrency issues and techniques to optimize database performance while managing reader-writer interactions.
READPAST & Furious - Transactions, Locking and Isolation. PASS Summit 2012 Edition
- 1. READPAST & Furious: Transactions, Locking and Isolation Mark Broadbent Senior SQL Specialist SQLCloud SQLCLOUD.CO.UK November 6-9, Seattle, WA
- 2. About Mark Broadbent “30 billion times more intelligent than a live mattress” Email: mark.broadbent@sqlcambs.org.uk Twitter: retracement Blog: http://tenbulls.co.uk Event Lead to the UK’s first ever SQLSaturday (Cambridge) http://www.sqlsaturday.com/events.aspx Cambridgeshire PASS Chapter UG Leader http://sqlcambs.org.uk DBA-309
- 3. Agenda TRANSACTIONS Structure, Scope, Management, the weird and the wonderful! LOCKING Compatibility, Multi Granularity and Escalation NOLOCK, READPAST and UPDLOCK ISOLATION Isolation Levels Snapshot Isolation and Read Committed Snapshot Rolling Database Snapshots AlwaysOn Readable Secondaries DBA-309
- 4. TRANSACTIONS Part I November 6-9, Seattle, WA
- 5. Transactions can be… Batch Scope Auto-Commit UPDATE … INSERT … MSDTC MSDTC INSERT … DELETE … DELETE … GO Distributed Explicit BEGIN TRAN … Binding Token spid 115 spid 162 Implicit (evil muthas!) BEGIN TRAN BEGIN TRAN UPDATE … INSERT … INSERT … BEGIN TRAN DELETE … Session Binding and provide Atomicity (all or nothing) …or do they?! They effect the duration of locks. It can be really confusing! DBA-309
- 6. Transactions are easy-peasy… Transaction Name Transaction Mark BEGIN TRAN transaction_1 WITH MARK ‘restorepoint’ BEGIN TRAN --do something COMMIT TRAN Savepoint SAVE TRAN savepoint BEGIN TRAN transaction_3 BEGIN TRAN Nested --do something else Transaction COMMIT IF {something_wrong} THEN ROLLBACK TRAN savepoint COMMIT COMMIT DBA-309
- 7. TRANSACTIONS Demo November 6-9, Seattle, WA
- 8. LOCKING Part II November 6-9, Seattle, WA
- 9. Locks, the Lock Manager and Locking Locks are ONLY memory structures • They can be converted or escalated • Lock escalation will occur for performance and memory savings • Are compatible OR incompatible with other locks • Will wait if they are incompatible • Are taken depending upon the Isolation level • Can cause blocking or deadlocks Lock Manager compares locks ONLY on same resource AND lock partition • So that’s why intent locks are needed (for granularity) DBA-309
- 10. DBA-309
- 11. NL SCH-S SCH-M S U X IS IU IX SIU SIX UIX BU RS-S RS-U RI-N RI-S RI-U RI-X RX-S RX-U RX-X NL SCH-S SCH-M S U X IS IU IX SQL Server SIU Lock Compatibility Chart SIX UIX BU RS-S RS-U RI-N RI-S No Conflict RI-U Conflict RI-X RX-S Illegal RX-U RX-X NL No Lock IX Intent Exclusive RI-S Insert Range-Shared SCH-S Schema Stability Lock SIU Share with Intent Update RI-U Insert Range-Update SCH-M Schema Modification Lock SIX Share with Intent Exclusive RI-X Insert Range-Exclusive S Shared UIX Update with Intent Exclusive RX-S Exclusive Range-Shared U Update BU Bulk Update RX-U Exclusive Range-Update X Exclusive RS-S Shared Range-Shared RX-X Exclusive Range-Exclusive IS Intent Shared RS-U Shared Range-Update IU Intent Update RI-N Insert Range-Null DBA-309
- 12. READPAST & Furious: Locking Blocking and Isolation · Mark Broadbent · sqlcloud.co.uk NL SCH-S SCH-M S U X IS IU IX SIU SIX UIX BU RS-S RS-U RI-N RI-S RI-U RI-X RX-S RX-U RX-X NL SCH-S SCH-M S U X IS IU IX SQL Server SIU Lock Compatibility Chart SIX UIX BU RS-S RS-U RI-N RI-S Compatible RI-U Incompatible RI-X RX-S Illegal RX-U RX-X NL No Lock IX Intent Exclusive RI-S Insert Range-Shared SCH-S Schema Stability Lock SIU Share with Intent Update RI-U Insert Range-Update SCH-M Schema Modification Lock SIX Share with Intent Exclusive RI-X Insert Range-Exclusive S Shared UIX Update with Intent Exclusive RX-S Exclusive Range-Shared U Update BU Bulk Update RX-U Exclusive Range-Update X Exclusive RS-S Shared Range-Shared RX-X Exclusive Range-Exclusive IS Intent Shared RS-U Shared Range-Update IU Intent Update RI-N Insert Range-Null http://bit.ly/sc9f7l DBA-309
- 13. Intent Locks IX Table orders Data Page IX X Row Free Space X DBA-309
- 14. Intent Locks Lock must wait! IX S Table orders Data Page IX X Row Free Space X DBA-309
- 15. Escalation Don’t escalate by TF1211 or take artificial IS on table Table Ignore # locks by TF1224 IX orders Data Page IX Partition X Row Free Space X ALTER <table> SET (LOCK_ESCALATION = auto| table| disable) DBA-309
- 16. Escalation Don’t escalate by TF1211 or take artificial IS on table Table Ignore # locks by TF1224 IX X orders Data Page Partition Row Free Space ALTER <table> SET (LOCK_ESCALATION = auto| table| disable) DBA-309
- 17. NOLOCK (a Wolf in Sheep’s Clothing?) Should use READUNCOMMITTED hint instead –but ONLY in very specialist situations. Doesn’t acquire Shared locks so is able to perform DIRTY READS, NON-REPEATABLE READS, PHANTOMS …it can also return DUPLICATE READS! It DOES NOT give “Oracle style” concurrency When can this ever be acceptable? DBA-309
- 18. Well since you mention it… AlwaysOn Readable Secondaries Scalable Shared Database “There is SQL Server NOLOCK” Instance A SQL Server Instance B Instance C SQL Server Instance B SQL Server Instance A Filegroup1 Filegroup2 Filegroup3 Readonly Database Database Snapshot Readonly Filegroup/s Blog post: “When should you use NOLOCK?“ http://bit.ly/rdGzow DBA-309
- 19. READPAST & (your boss) may be Furious! Is an alternative to NOLOCK/ READUNCOMMITTED. Skips over resources holding incompatible (to S) locks. …does not therefore cause dirty or duplicate reads. (yay!) DOES NOT give “Oracle style” concurrency …and can return INCOMPLETE DATA SETS! When can this ever be acceptable? DBA-309
- 20. Using READPAST and Table Queues --Get the first record orders --possible and update UPDATE u SET … X X FROM 2 U ( SELECT TOP(1) * FROM orders U WITH (READPAST ,UPDLOCK) WHERE … 1 ) AS U DBA-309
- 21. LOCKING Demo November 6-9, Seattle, WA
- 22. ISOLATION Part III November 6-9, Seattle, WA
- 23. BAD DEPENDENCIES Pessimistic Non- Optimistic Update Dirty Read Repeatable Phantoms Conflict Read 1 READ UNCOMMITTED Warning! Warning! Warning! OK 2 READ COMMITTED OK Warning! Warning! OK …WITH SNAPSHOT OK Warning! Warning! OK 3 REPEATABLE READ OK OK Warning! OK 4 SERIALIZABLE OK OK OK OK 5 SNAPSHOT OK OK OK Warning! DBA-309
- 24. Read Committed Snapshot vs Snapshot Isolation Read Committed Snapshot Snapshot Isolation at the statement level. Isolation at the transaction level. Becomes the new default Must be explicitly SET in each Isolation. connection. Requires Exclusive Transaction Requires no active transactions in Workspace lock (i.e. no other order to transition. connections to DB). Is allowed master, tempdb & Not allowed on master, tempdb & msdb. msdb. Implements automatic update Conflict detection not required. conflict detection. DBA-309
- 25. Optimistic Concurrency #FAIL Snapshot Isolation introduces update conflicts! TempDB overhead for version store. Writers STILL block writers and their escalation can still cause a concurrency problem! RCSI (and SI) are not a silver bullet for Concurrency. DBA-309
- 26. Database Snapshots spid 115 5 COMMIT or COMMIT or spid 115 ROLLBACK ROLLBACK 1 BEGIN TRAN UPDATE spid 162 2 orders CREATE DATABASE ssdb SET … … WHERE … Buffer Cache AS SNAPSHOT OF db 3 4 Dirty Pages Automatic Recovery CHECKPOINT Datafile Transaction Log Blog post: “Lifting the lid on database snapshots“ http://bit.ly/NS64qA DBA-309
- 27. Readable Secondaries Great for reporting, but… Cause a 14 byte pointer to be added to Primary database –since Secondary uses SNAPSHOT isolation under the covers. Beware sizing considerations and page splitting. Data latency from redo thread can cause unexpected results if your application does not expect it. More administration overhead and knowledge. You cannot write to them (name is the giveaway!). DBA-309
- 28. ISOLATION Demo November 6-9, Seattle, WA
- 29. Summary Concurrency is a very complicated topic, ensure you understand the effects of running different transactional modes and Isolation levels. Understand the duration and compatibility of common locks and how they are effected. Isolation can be achieved through scalability mechanisms and techniques, your ultimate aim is to separate writers from readers, readers from writers and writers from writers. In short maximize lock compatibility.
- 30. References and Thanks Kalen Delaney, SQLPASS Summit 2011 DBA301P - Locking and Blocking and Row Versions, Oh My! – DVD Jose Barreto's Blog http://bit.ly/nPJGjv Paul White http://sqlblog.com/blogs/paul_white Benjamin Neverez http://www.benjaminnevarez.com/ DBA-309
- 31. PASS Resources Free SQL Server and BI training Free 1-day Training Events Regional Event Local and Virtual User Groups Free Online Technical Training This is Community Learning Center DBA-309
- 32. Thank you for attending this session and the 2012 PASS Summit in Seattle DBA-309 November 6-9, Seattle, WA