SQLSaturday 323 – Paris 2014
Welcome to the nightmare of locking, blocking and isolation levels!
SQLSaturday 323 – Paris 2014
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SQLSaturday 323 – Paris 2014
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SQLSaturday 323 – Paris 2014
So who am I?
@BorisHristov
So who am I?
SQLSaturday 323 – Paris 2014
Agenda…
Locks. What is there for us?
Troubleshooting locking problems
Transaction Isolation Levels
SQLSaturday 323 – Paris 2014
Locks. What is there for us?
SQLSaturday 323 – Paris 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…
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
Lock Hierarchy
Database
Table
Page
Row
SQLSaturday 323 – Paris 2014
Let’s update a row! What do we need?
USE AdventureWorks2012GOUPDATE [Person].[Address]SET AddressLine1=’Paris, France' WHERE AddressID=2
S
IX
HeaderRow
Row
Row
Row
Row
IX
X
SQLSaturday 323 – Paris 2014
Methods to View Locking Information
Dynamic Management
Views
SQL Server Profiler or
Extended Events
Performance monitor or
Activity Monitor
SQLSaturday 323 – Paris 2014
Troubleshooting locking problems
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
Lock escalation S
S
X
>= 5000
IX
Header
Row
Row
Row
Row
Row
X
X
X
IX
X
SQLSaturday 323 – Paris 2014
1. 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
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_descFROM sys.tablesWHERE name = 'Person.Address'
ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE | DISABLE}
SQLSaturday 323 – Paris 2014
What Are Deadlocks?
Task A
Task B
Resource 1
Resource 2
Who is victim?
• Cost for Rollback
• Deadlock priority – SET DEADLOCK_PRIORITY
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
DEMO Monitor for locks with xEvents
Lock escalation – both to table and partition
Deadlock and the SET DEADLOCK_PRIORITY option
SQLSaturday 323 – Paris 2014
Transaction isolation levels
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
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)
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
DEMO Playing around with the Isolation levels
SQLSaturday 323 – Paris 2014
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!
SQLSaturday 323 – Paris 2014
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
SQLSaturday 323 – Paris 2014
Merci!