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!

1. Welcome to the nightmare of
locking, blocking and isolation levels!
November 9th, 2013
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2. So who am I?
@BorisHristov
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3. Agenda…
Locks. What is there for us?
Troubleshooting locking problems
Transaction Isolation Levels
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4. Locks. What is there for us?
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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…
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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
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7. Common lock types
Shared (S)
Update (U)
Used for: Reading
Duration: Released almost immediately
(depends on the isolation level)
Used for: Preparing to modify
Duration: End of the transaction or until
converted to exclusive (X)
Exclusive (X)
Intent
Used for: Modifying
Duration: End of the transaction
Used for: Preventing incompatible locks
Duration: End of the transaction
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8. Lock Compatibility
Not all locks are compatible with other locks.
Lock
Shared
Update
Exclusive

X
Update
(U)


X
X
Exclusive
(X)
X
X
X
Shared
(S)
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9. Lock Hierarchy
Database
Table
Page
Row
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10. Let’s update a row!
What do we need?
S
A query! 
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Verona,Italy'
WHERE AddressID=2
IX
IX
Header
Row
Row
Row
Row
X
Row
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11. Methods to View Locking Information
Dynamic
Management
Views
November 9th, 2013
SQL Server
Profiler or
Extended
Events
Performance
monitor or Activity
Monitor
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12. Troubleshooting locking problems
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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
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14. Lock granularity and escalation
SQL Server decides (during compilation) the granularity of locks to be used:
•
•
Row
Page
Lock granularity can be controlled via LOCK HINTS (PAGLOCK, TABLOCKX…)
Escalation always happens this way:
Row -> table lock (or partition lock if possible)
Page -> table lock (or partition lock if possible)
Lock escalation can be disabled:
•
•
Trace flag 1211 – disables lock escalation on server level
Trace flag 1224 – disables lock escalation on server level until 40% of the
memory used is consumed
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15. Lock escalation
S
S
IX
X
IX
Header
Row
Row
X
X
Row
Row
X
Row
>= 5000
X
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16. Controlling Lock escalation
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
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17. What Are Deadlocks?
Who is victim?
• Cost for Rollback
• Deadlock priority – SET DEADLOCK_PRIOIRTY
Task A
Task B
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Resource 1
Resource 2
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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
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19. DEMO
Monitor for locks with xEvents
Lock escalation – both to table and partition
Deadlock and the SET DEADLOCK_PRIORITY option
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20. Transaction isolation levels
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21. Read Uncommitted
(pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?)
Transaction 1
eXclusive lock
Update
Select
Transaction 2
Dirty read
Suggestion: Better offload the reads or go with optimistic level concurrency!
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22. Repeatable Read
(pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Transaction 1
S(hared) lock
select
Transaction 2
Update
No non-repeatable reads possible (updates during Transaction 1)
Phantom records still possible (inserts during Transaction 1)
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23. Serializable
(pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
Transaction 1
S(hared) lock
select
Transaction 2
Insert
Even phantom records are not possible!
Highest pessimistic level of isolation, lowest level of concurrency
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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
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25. RCSI and SI
(optimistic concurrency control)
V1
Transaction 1
Select
Transaction 2
V2
Select in RCSI
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
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26. DEMO
Playing around with the Isolation levels
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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!
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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-serverlocking-with-hints/5181472
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29. Thank you.
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