Download to read offline
![Let’s update a row!
What do we need?
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Paris, France'
WHERE AddressID=2
SQLSaturday 323 – Paris 2014
S
IX
Header
Row
Row
Row
Row
Row
IX
X](https://image.slidesharecdn.com/sqlsaturday323paris2014slidedeck-140913071001-phpapp01/75/The-Nightmare-of-Locking-Blocking-and-Isolation-Levels-12-2048.jpg)
Uploaded byBoris Hristov
The Nightmare of Locking, Blocking and Isolation Levels
The document discusses SQL Server locking, blocking, and isolation levels presented at SQLSaturday 323 in Paris. It explains different types of locks, issues like deadlocks and lock escalation, and strategies for controlling these problems, including choosing appropriate transaction isolation levels. The importance of monitoring locking behavior and making informed decisions about concurrency control is emphasized throughout the presentation.
More Related Content
Similar to The Nightmare of Locking, Blocking and Isolation Levels
The Nightmare of Locking, Blocking and Isolation Levels
- 1. Welcome to thenightmare of locking, blocking and isolation levels! SQLSaturday 323 – Paris 2014
- 2. Sponsors Gold SQLSaturday323 – Paris 2014
- 3. Sponsors Silver etBronze SQLSaturday 323 – Paris 2014
- 4. So who amI? So who am I? @BorisHristov SQLSaturday 323 – Paris 2014
- 5. Agenda… Locks. Whatis there for us? Troubleshooting locking problems Transaction Isolation Levels SQLSaturday 323 – Paris 2014
- 6. Locks. What isthere for us? SQLSaturday 323 – Paris 2014
- 7. Methods of ConcurrencyControl 1. Pessimistic – SQL Server uses locks, causes blocks and who said deadlocks? 2. Optimistic – SQL Server generates versions for everyone, but the updates… SQLSaturday 323 – Paris 2014
- 8. What Are Locksand 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 SQLSaturday 323 – Paris 2014
- 9. Common lock types SQLSaturday 323 – Paris 2014 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
- 10. Lock Compatibility Notall locks are compatible with other locks. Lock Shared Update Exclusive Shared X (S) Update (U) X X Exclusive (X) X X X SQLSaturday 323 – Paris 2014
- 11. Lock Hierarchy SQLSaturday323 – Paris 2014 Database Table Page Row
- 12. Let’s update arow! What do we need? USE AdventureWorks2012 GO UPDATE [Person].[Address] SET AddressLine1=’Paris, France' WHERE AddressID=2 SQLSaturday 323 – Paris 2014 S IX Header Row Row Row Row Row IX X
- 13. Methods to ViewLocking Information Dynamic Management Views SQL Server Profiler or Extended Events SQLSaturday 323 – Paris 2014 Performance monitor or Activity Monitor
- 14. Troubleshooting locking problems SQLSaturday 323 – Paris 2014
- 15. 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 SQLSaturday 323 – Paris 2014
- 16. Lock escalation S IX X SQLSaturday 323 – Paris 2014 S X >= 5000 IX Header Row Row Row Row Row X X X
- 17. 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 SQLSaturday 323 – Paris 2014
- 18. What Are Deadlocks? Task A Task B SQLSaturday 323 – Paris 2014 Resource 1 Resource 2 Who is victim? • Cost for Rollback • Deadlock priority – SET DEADLOCK_PRIORITY
- 19. Resolve blocking a.k.alive 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 SQLSaturday 323 – Paris 2014
- 20. DEMO Monitor forlocks with xEvents Lock escalation – both to table and partition Deadlock and the SET DEADLOCK_PRIORITY option SQLSaturday 323 – Paris 2014
- 21. Transaction isolation levels SQLSaturday 323 – Paris 2014
- 22. Read Uncommitted (pessimisticconcurrency 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! SQLSaturday 323 – Paris 2014
- 23. Repeatable Read (pessimisticconcurrency control) 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) SQLSaturday 323 – Paris 2014 Update Transaction 2
- 24. Transaction 1 S(hared)lock select Even phantom records are not possible! Highest pessimistic level of isolation, lowest level of concurrency SQLSaturday 323 – Paris 2014 Insert Transaction 2 Serializable (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
- 25. Optimistic Concurrency Basedon 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 SQLSaturday 323 – Paris 2014
- 26. RCSI and SI (optimistic concurrency control) Transaction 1 V1 V2 Select Select in RCSI Transaction 2 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 SQLSaturday 323 – Paris 2014 Select in SI
- 27. DEMO Playing aroundwith the Isolation levels SQLSaturday 323 – Paris 2014
- 28. Summary 1. Blockingis 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! SQLSaturday 323 – Paris 2014
- 29. Resources MCM Readinessvideos 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 SQLSaturday 323 – Paris 2014
- 30. Merci! SQLSaturday 323– Paris 2014