The nightmare of locking, blocking and isolation levels!

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am sure you all know that troubleshooting problems related to locking and blocking (hey, sometimes there are deadlocks too) can be a real nightmare! In this session, you will be able to see and understand why and how locking actually works, what problems it causes and how can we use isolation levels and various other techniques to resolve them!

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The nightmare of locking, blocking and isolation levels!

  1. 1. The Nightmare of Locking, Blocking and Isolation Levels
  2. 2. So who am I? @BorisHristov So who am I?
  3. 3. Agenda… Locks. What is there for us? Troubleshooting locking problems Transaction Isolation Levels
  4. 4. Locks. What is there for us?
  5. 5. 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…
  6. 6. 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
  7. 7. Common lock types Intent Used for: Preventing incompatible locks Duration: End of the transaction Shared (S) Used for: Reading Duration: Released almost immediately (depends on the isolation level) Update (U) Used for: Preparing to modify Duration: End of the transaction or until converted to exclusive (X) Exclusive (X) Used for: Modifying Duration: End of the transaction
  8. 8. Lock Compatibility Not all locks are compatible with other locks. Lock Shared Update Exclusive Shared (S)   X Update (U)  X X Exclusive (X) X X X
  9. 9. Lock Hierarchy Database Table Page Row
  10. 10. Let’s update a row! What do we need? USE AdventureWorks2012 GO UPDATE [Person].[Address] SET AddressLine1=’Zagreb,Croatia' WHERE AddressID=2 S IX Header Row Row Row Row Row IX X
  11. 11. How to View Locking Information Dynamic Management Views SQL Server Profiler or Extended Events Performance monitor or Activity Monitor
  12. 12. Troubleshooting Locking Problems
  13. 13. 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
  14. 14. Lock escalationS S X >= 5000 IX Header Row Row Row Row Row X X X IX X
  15. 15. 1. Switch the escalation level (per table) 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 Controlling Lock escalation SELECT lock_escalation_desc FROM sys.tables WHERE name = 'Person.Address' ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE | DISABLE}
  16. 16. What Are Deadlocks? Task A Task B Resource 1 Resource 2 Who is victim? • Cost for Rollback • Deadlock priority – SET DEADLOCK_PRIOIRTY
  17. 17. 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
  18. 18. DEMO Monitor for locks with xEvents Lock escalation – both to table and partition Deadlock and the SET DEADLOCK_PRIORITY option
  19. 19. Transaction Isolation Levels
  20. 20. SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?) Transaction 1 Transaction 2 Suggestion: Better offload the reads or go with optimistic level concurrency! Select Update eXclusive lock Read Uncommitted (pessimistic concurrency control) Dirty read
  21. 21. SET TRANSACTION ISOLATION LEVEL REPEATABLE READ Transaction 1 S(hared) lock select No non-repeatable reads possible (updates during Transaction 1) Phantom records still possible (inserts during Transaction 1) Update Transaction 2 Repeatable Read (pessimistic concurrency control)
  22. 22. Transaction 1 S(hared) lock select Even phantom records are not possible! Highest pessimistic level of isolation, lowest level of concurrency Insert Transaction 2 Serializable (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
  23. 23. 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 Optimistic Concurrency
  24. 24. 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 RCSI and SI (optimistic concurrency control) V1 V2 Transaction 1 Transaction 2 Select in RCSISelect Select in SI
  25. 25. DEMO Playing around with the Isolation levels
  26. 26. Summary 1. Blocking is something normal when it’s not for long 2. There are numerous ways to monitor locking and blocking 3. Be extremely careful for lock escalations 4. Choosing the Isolation level is also a business decision!
  27. 27. 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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