The nightmare of locking, blocking and deadlocking. SQLSaturday #257, Verona

#sqlsat257#sqlsatveronaNovember 9th, 2013

Welcome to the nightmare of locking, blocking and isolation levels!


So who am I?

@BorisHristov

http://www.cc.nbu.bg/


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 – SQL Server generates versions for everyone, but the updates…


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


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


Lock Compatibility

Not all locks are compatible with other locks.

Lock Shared Update Exclusive

Shared (S) XUpdate (U) X X

Exclusive (X) X X X

http://technet.microsoft.com/en-us/library/ms186396(v=sql.105).aspx


Lock Hierarchy

Database

Table

Page

Row


Let’s update a row! What do we need?

USE AdventureWorks2012GOUPDATE [Person].[Address]SET AddressLine1=’Verona,Italy' WHERE AddressID=2

S

IX

HeaderRow

Row

Row

Row

Row

IX

X

A query!


Methods to View Locking Information

Dynamic Management

Views

SQL Server Profiler or Extended

Events

Performance monitor or Activity

Monitor


Troubleshooting locking problems


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


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

Lock granularity and escalation


Lock escalation S

S

X

>= 5000

IX

Header

Row

Row

Row

Row

Row

X

X

X

IX

X


Switch the escalation level (per table)

AUTO – Partition-level escalation if the table is partitionedTABLE – Always table-level escalationDISABLE – Do not escalate until absolutely necessary

Controlling Lock escalation

SELECT lock_escalation_descFROM sys.tablesWHERE name = 'Person.Address'

ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE | DISABLE}


What Are Deadlocks?

Task A

Task B

Resource 1

Resource 2

Who is victim?

• Cost for Rollback

• Deadlock priority – SET DEADLOCK_PRIOIRTY


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


DEMO Monitor for locks with xEvents

Lock escalation – both to table and partition

Deadlock and the SET DEADLOCK_PRIORITY option


Transaction isolation levels


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


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)


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


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


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


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 blocking

3. Be extremely careful for lock escalations

4. Choosing the Isolation level is also a business decision!


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

http://technet.microsoft.com/en-US/sqlserver/gg508892.aspx

http://technet.microsoft.com/en-us/sqlserver/gg508891.aspx



http://blogs.msdn.com/b/bartd/archive/tags/sql+locking






http://www.techrepublic.com/article/control-sql-server-locking-with-hints/5181472




Thank you.

Technology

The nightmare of locking, blocking and deadlocking. SQLSaturday #257, Verona