The Nightmare of Locking, Blocking and Isolation Levels!
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This document provides an overview of locking, blocking, and transaction isolation levels in SQL Server. It begins with an introduction to locking and how SQL Server uses locks for concurrency control. It then discusses the different lock types, lock compatibility, and the lock hierarchy. The document demonstrates how to view locking information and troubleshoot locking problems. It also covers lock escalation, deadlocks, and resolving blocking issues. Finally, it concludes with an explanation of the various transaction isolation levels supported by SQL Server and how they differ in terms of concurrency and consistency.
![Let’s update a row!
What do we need?
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Dublin, Ireland'
WHERE AddressID=2
S
IX
IX
Header
Row
Row
Row
Row
Row
X](https://image.slidesharecdn.com/slidedeckfinal-140920230939-phpapp01/85/The-Nightmare-of-Locking-Blocking-and-Isolation-Levels-11-320.jpg)
The Nightmare of Locking, Blocking and Isolation Levels!
- 1. Welcome to the nightmare of locking, blocking and isolation levels!
- 2. With thanks to our sponsors
- 3. So who am I? So who am I? @BorisHristov
- 4. Agenda… Locks. What is there for us? Troubleshooting locking problems Transaction Isolation Levels
- 5. Locks. What is there for us?
- 6. Methods of Concurrency Control 1. Pessimistic – SQL Server uses locks, causes blocks and who said deadlocks? 2. Optimistic – SQL Server generates versions for everyone, but the updates…
- 7. What Are Locks and what is locking? Lock – internal memory structure that “tells” us what we all do with the resources inside the system Locking – mechanism to protect the resources and guarantee consistent data
- 8. Common lock types Intent Used for: Preven-ng incompa-ble locks Dura-on: End of the transac-on Shared (S) Used for: Reading Dura-on: Released almost immediately (depends on the isola-on level) Update (U) Used for: Preparing to modify Dura-on: End of the transac-on or un-l converted to exclusive (X) Exclusive (X) Used for: Modifying Dura-on: End of the transac-on
- 9. Lock Compatibility Not all locks are compatible with other locks. Lock Shared Update Exclusive Shared (S) P P X Update (U) P X X Exclusive (X) X X X
- 10. Lock Hierarchy Database Table Page Row
- 11. Let’s update a row! What do we need? USE AdventureWorks2012 GO UPDATE [Person].[Address] SET AddressLine1=’Dublin, Ireland' WHERE AddressID=2 S IX IX Header Row Row Row Row Row X
- 12. Methods to View Locking Information Dynamic Management Views SQL Server Profiler or Extended Events Performance monitor or Ac-vity Monitor
- 14. Locking and blocking Locking and blocking are often confused! Locking • The action of taking and potentially holding locks • Used to implement concurrency control Blocking is result of locking! • One process needs to wait for another process to release locked resources • In a multiuser environment, there is always, always blocking! • Only a problem if it lasts too long
- 15. Lock escalation S S X >= 5000 IX Header Row Row Row Row Row IX X X X X
- 16. Controlling Lock escalation 1. Switch the escalation level (per table) SELECT lock_escalation_desc FROM sys.tables WHERE name = 'Person.Address' ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE | DISABLE} AUTO – Partition-level escalation if the table is partitioned TABLE – Always table-level escalation DISABLE – Do not escalate until absolutely necessary 2. Just disable it (that’s not Nike’s “Just do it!”) • Trace flag 1211 – disables lock escalation on server level • Trace flag 1224 – disables lock escalation if 40% of the memory used is consumed
- 17. What Are Deadlocks? Task A Task B Resource 1 Resource 2 Who is victim? • Cost for Rollback • Deadlock priority – SET DEADLOCK_PRIORITY
- 18. Resolve blocking a.k.a live locking 1. Keep the transactions as short as possible 2. No user interactions required in the middle of the transaction 3. Use indexes (proper ones) 4. Consider a server to offload some of the workloads 5. Choose isolation level
- 19. DEMO Monitor for locks with xEvents Lock escalation – both to table and partition Deadlock and the SET DEADLOCK_PRIORITY option
- 21. Read Uncommitted (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?) Transaction 1 Select eXclusive lock Transaction 2 Update Dirty read Suggestion: Better offload the reads or go with optimistic level concurrency!
- 22. Repeatable Read (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL REPEATABLE READ Transaction 1 S(hared) lock select Update Transaction 2 No non-repeatable reads possible (updates during Transaction 1) Phantom records still possible (inserts during Transaction 1)
- 23. (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE Transaction 1 S(hared) lock select Serializable Insert Transaction 2 Even phantom records are not possible! Highest pessimistic level of isolation, lowest level of concurrency
- 24. Optimistic Concurrency Based on Row versioning (stored inside tempdb’s version store area) • No dirty, non-repeatable reads or phantom records • Every single modification is versioned even if not used • Adds 14 bytes per row Readers do not block writers and writers do not block readers Writers can and will block writers, this can cause conflicts
- 25. RCSI and SI (optimistic concurrency control) Transaction 1 V1 V2 Select Select in RCSI Transaction 2 Select in SI RCSI – Read Committed Snapshot Isolation Level • Statement level versioning • Requires ALTER DATABASE SET READ_COMMITTED_SNAPSHOT ON Snapshot Isolation Level • Transaction level versioning • Requires ALTER DATABASE SET ALLOW_SNAPSHOT_ISOLATION ON • Requires SET TRANSACTION ISOLATION LEVEL SNAPSHOT
- 26. DEMO Playing around with the Isolation levels
- 27. Summary 1. Blocking is something normal when it’s not for long 2. There are numerous of ways to monitor locking and blocking 3. Be extremely careful for lock escalations 4. Choosing the Isolation level is also a business decision!
- 28. Resources MCM Readiness videos on locking lecture and demo MCM Readiness video on Snapshot Isolation Level http://blogs.msdn.com/b/bartd/archive/tags/sql+locking http://www.sqlskills.com/blogs/paul/category/locking/ Lock hints - http://www.techrepublic.com/article/control-sql-server-locking- with-hints/5181472
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- 30. 10:10 - Next Session … § Lady Windermere (Charlemont #1) § Carmel Gunn & Bob Duffy – The Irish Economic Crisis Visualized with Power BI § Dorian Gray (Charlemont #2) § Mladen Prajdic – Digging into the .NET SQLClient § Intentions (Charlemont #3) § Chris Webb – Power Query: Beyond the Basics § Ideal Husband (Charlemont #4) § Ewan Fairweather – Shared technical learning for building highly scalable & available cloud apps § Lord Arthur Saville (Charlemont #5) § William Durkin – Replication Troubleshooting & Monitoring
- 31. Thank you!