the nightmare of locking, blocking and isolation levels!
TRANSCRIPT
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Boris Hristov
SQL Server Consultant and Trainer
Welcome to the nightmare of locking,
blocking and isolation levels!
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@BorisHristov
So who am I?
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Agenda…
Locks. What is there for us?
Troubleshooting locking problems
Transaction Isolation Levels
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Locks. What is there for us?
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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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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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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
Common Lock Types
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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
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Lock Hierarchy
Database
Table
Page
Row
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Let’s update a row!
What do we need?
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Belgrade, Serbia'
WHERE AddressID=2
S
IX
Header
Row
Row
Row
Row
Row
IX
X
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Methods to View Locking Information
Dynamic
Management
Views
SQL Server
Profiler or
Extended
Events
Performance
Monitor or
Activity Monitor
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Troubleshooting locking problems
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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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Lock escalationS
S
X
>= 5000
IX
Header
Row
Row
Row
Row
Row
X
X
X
IX
X
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Controlling Lock Escalation
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
SELECT lock_escalation_desc
FROM sys.tables
WHERE name = 'Person.Address'
ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE |
DISABLE}
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What Are Deadlocks?
Task A
Task B
Resource 1
Resource 2
Who is victim?
• Cost for Rollback
• Deadlock priority – SET DEADLOCK_PRIORITY
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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
Resolving blocking a.k.a live locking
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DEMO Monitor for locks with xEvents
Lock escalation – both to table and partition
Deadlock and the SET DEADLOCK_PRIORITY option
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Transaction isolation levels
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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
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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)
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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
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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
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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 V2Transaction 1
Transaction 2
Select in RCSISelect
Select in SI
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DEMO Playing around with the Isolation levels
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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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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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Thank you!
Boris Hristov
www.borishristov.com