Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.
Welcome to the nightmare of
locking, blocking and isolation levels!
Boris Hristov
Sponsors
Gold Sponsors:
Bronze Sponsors:
Swag Sponsors:
So who am I?
@BorisHristov
So who am I?
Agenda…
Locks. What is there for us?
Troubleshooting locking problems
Transaction Isolation Levels
Locks. What is there for us?
Methods of Concurrency Control
1. Pessimistic
– SQL Server uses locks, causes blocks and who said deadlocks?
2. Optimistic...
What Are Locks and what is locking?
Lock – internal memory structure that “tells” us what we all do with the
resources ins...
Common lock types
Intent
Used for: Preventing incompatible locks
Duration: End of the transaction
Shared (S)
Used for: Rea...
Lock Compatibility
Not all locks are compatible with other locks.
Lock Shared Update Exclusive
Shared
(S)   X
Update
(U)...
Lock Hierarchy
Database
Table
Page
Row
Let’s update a row!
What do we need?
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Varna, Bulgaria...
Methods to View Locking Information
Dynamic
Management
Views
SQL Server
Profiler or
Extended Events
Performance
monitor or...
Troubleshooting locking problems
Locking and blocking
Locking and blocking are often confused!
Locking
• The action of taking and potentially holding locks...
Lock escalation
S
S
X
>= 5000
IX
Header
Row
Row
Row
Row
Row
X
X
X
IX
X
1. Switch the escalation level (per table)
AUTO – Partition-level escalation if the table is partitioned
TABLE – Always ta...
What Are Deadlocks?
Task A
Task B
Resource 1
Resource 2
Who is victim?
• Cost for Rollback
• Deadlock priority – SET DEADL...
Resolve blocking a.k.a live locking
1. Keep the transactions as short as possible
2. No user interactions required in the ...
DEMO
Monitor for locks with xEvents
Lock escalation – both to table and partition
Deadlock and the SET DEADLOCK_PRIORITY o...
Transaction isolation levels
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?)
Transaction 1
Transaction 2
Suggestion: Better offload the read...
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Transaction 1 S(hared) lock
select
No non-repeatable reads possible (updat...
Transaction 1 S(hared) lock
select
Even phantom records are not possible!
Highest pessimistic level of isolation, lowest l...
Based on Row versioning (stored inside tempdb’s version store area)
• No dirty, non-repeatable reads or phantom records
• ...
RCSI – Read Committed Snapshot Isolation Level
• Statement level versioning
• Requires ALTER DATABASE SET READ_COMMITTED_S...
DEMO
Playing around with the Isolation levels
Summary
1. Blocking is something normal when it’s not for long
2. There are numerous of ways to monitor locking and blocki...
Resources
MCM Readiness videos on locking lecture and demo
MCM Readiness video on Snapshot Isolation Level
http://blogs.ms...
Thank you!
Contacts:
brshristov@live.com
@BorisHristov
www.borishristov.com
Upcoming SlideShare
Loading in …5
×

The Nightmare of Locking, Blocking and Isolation Levels!

1,861 views

Published on

I 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!

Published in: Technology
  • Be the first to comment

The Nightmare of Locking, Blocking and Isolation Levels!

  1. 1. Welcome to the nightmare of locking, blocking and isolation levels! Boris Hristov
  2. 2. Sponsors Gold Sponsors: Bronze Sponsors: Swag Sponsors:
  3. 3. So who am I? @BorisHristov So who am I?
  4. 4. Agenda… Locks. What is there for us? Troubleshooting locking problems Transaction Isolation Levels
  5. 5. Locks. What is there for us?
  6. 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. 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. 8. 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
  9. 9. 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
  10. 10. Lock Hierarchy Database Table Page Row
  11. 11. Let’s update a row! What do we need? USE AdventureWorks2012 GO UPDATE [Person].[Address] SET AddressLine1=’Varna, Bulgaria' WHERE AddressID=2 S IX Header Row Row Row Row Row IX X
  12. 12. Methods to View Locking Information Dynamic Management Views SQL Server Profiler or Extended Events Performance monitor or Activity Monitor
  13. 13. Troubleshooting locking problems
  14. 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. 15. Lock escalation S S X >= 5000 IX Header Row Row Row Row Row X X X IX X
  16. 16. 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}
  17. 17. What Are Deadlocks? Task A Task B Resource 1 Resource 2 Who is victim? • Cost for Rollback • Deadlock priority – SET DEADLOCK_PRIORITY
  18. 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. 19. DEMO Monitor for locks with xEvents Lock escalation – both to table and partition Deadlock and the SET DEADLOCK_PRIORITY option
  20. 20. Transaction isolation levels
  21. 21. 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
  22. 22. 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)
  23. 23. Transaction 1 S(hared) lock select Even phantom records are not possible! Highest pessimistic level of isolation, lowest level of concurrency Oh, TransactionScope in C# default to this level too Insert Transaction 2 Serializable (pessimistic concurrency control) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
  24. 24. 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
  25. 25. 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
  26. 26. DEMO Playing around with the Isolation levels
  27. 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. 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
  29. 29. Thank you! Contacts: brshristov@live.com @BorisHristov www.borishristov.com

×