13.12.2014

Welcome to the nightmare of

locking, blocking and isolation levels!

Boris Hristov

13.12.2014

Thank you to all our SPONORS!

13.12.2014

So who am I?

@BorisHristov

So who am I?

13.12.2014

Agenda…

Locks. What is there for us?

Troubleshooting locking problems

Transaction Isolation Levels

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Locks. What is there for us?

13.12.2014

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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Common lock types

Intent

Used for: Preventing incompatible locksDuration: End of the transaction

Shared (S)

Used for: ReadingDuration: Released almost immediately

(depends on the isolation level)

Update (U)

Used for: Preparing to modifyDuration: End of the transaction or until

converted to exclusive (X)

Exclusive (X)

Used for: ModifyingDuration: End of the transaction

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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=’Ljubljana, Slovenia'

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 escalation

S

S

X

>= 5000

IX

Header

Row

Row

Row

Row

Row

X

X

X

IX

X

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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}

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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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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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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 READTransaction 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

13.12.2014

Hvala!

Contacts:

brshristov@live.com

@BorisHristov

www.borishristov.com