Does the Optimistic Concurrency resolve your blocking problems? Margarita Naumova, SQL Master Academy

Size: px

Start display at page:

Download "Does the Optimistic Concurrency resolve your blocking problems? Margarita Naumova, SQL Master Academy"

Madison Hudson
6 years ago
Views:

2 Does the Optimistic Concurrency resolve your blocking problems? Margarita Naumova, SQL Master Academy

MAGI NAUMOVA Working with SQL Server from v6.

projects in Bulgaria, Finland, Germany, UK, Sweden 5

Microsoft Worldwide Technical Leadership Team BG SQL

Server, running SQL Master Academy Training Program -

3 MAGI NAUMOVA Working with SQL Server from v6.5 SQL Server Trainer and Consultant with over 60 projects in Bulgaria, Finland, Germany, UK, Sweden 5 years Senior Consultant in Microsoft, member of Microsoft Worldwide Technical Leadership Team BG SQL UG Founder and Leader Now teaching and presenting SQL Server, running SQL Master Academy Training Program - and SQL Server consulting company -

4 Approaching the blocking today The blocking and how to isolate the reason The common blocking types and what we are doing wrong The Optimistic Concurrency how it works, what to take into account What does and what doesn t the OC resolve

5 DEMO: ISOLATE THE PROBLEM

6 The common reasons of blocking Writers block readers and vise versa Resource blocking i.e. transaction performance Lock duration Lock escalation Holding exclusive lock too early Increasing isolation level to RR because the need of consistent reading Serializable lock escalation on inserts (no index or using EF) Schema blocking Writers block writers

7 How we usually deal with blocking in pessimistic concurrency Controlling lock escalation ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO TABLE DISABLE) 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 Controlling Lock duration with SET LOCK TIMEOUT n(sec) Error 1222!!! Reading dirty data with hints NO_LOCK, READPAST Data consistency!!

8 How we usually deal with blocking in pessimistic concurrency Solutions design changes Index analysis and changing indexes works in some cases Partitioning Re-writing cursors logic and long running transactions Readers and Writers workload separation Switch to optimistic concurrency model In Memory OLTP tables in SQL Server 2014

9 Optimistic Concurrency Based on Row versioning When updating a row, the previous version is stored in the version store The new row contains a pointer to the old row in the version store V1 Transaction 1 Select V2 Transaction 2 Adds 14bytes to every row Needs Version Store i.e. TempDB space Readers do not block writers and writers do not block readers. BUT writers can and will block writers, this can cause conflicts we still have locks!

10 Implementation RCSI and SI Read Committed Snapshot Isolation Statement-Level versioning, i.e. any query sees the last version of data as of the beginning of the statement V1 Transaction 1 Select V2 Select in RCSI Transaction 2 Select in SI Snapshot Isolation Level Session Level versioning If used then better together with RCSI! Else, you have drawbacks of versioning only! Conflicts occur!! Some DDL statements are not allowed within SI transactions (no metadata versioning exists)

11 DEMO: OPTIMISTIC CONCURRENCY

12 Choosing between Optimistic Concurrency Models Tempdb space RCSI Consumes less tempdb space SI More tempdb space is required Distributed transaction X Update Conflicts NO update conflicts Update conflicts are possible Changes to app Repeatable read consistency inside tran No changes in cases you use default IL NO Require Changes to session IL inside the code YES

13 What problems the Optimistic Concurrency resolves? SELECT operations do not acquire shared locks Writers block readers and vise versa Lock escalation of S locks Lock duration if the concurrent transaction is a reader Holding exclusive lock too early if the concurrent transaction is a reader Reducing concurrency because the need for repeatable read consistency inside a transaction Lock escalation on inserts (Serializable) with some assumptions

14 What problems the optimistic concurrency doesn t resolve? Resource blockings you need to perform resource optimizations first Schema blocking 2014 OIR in case of maintenance and managed lock priority for partition switching In all other cases perform schema changes offline with proper planning Writers block writers Accept either blocking or conflict s Lock escalation of X locks Plan some solution design changes Latch contention special blocking case

15 Latch contention special blocking case

16 DEMO: LATCH CONTENTION

17 What did we cover? The need to find the root cause of your blocking problem Optimistic Concurrency resolves Readers blocking Writers problem only with a price! In all other cases you should be considering another solution SQL Server 2014 InMemory OLTP could be the silver bullet in your case in locking and latch contention for high intensive OLTP workloads!

18 THANK YOU! For attending this session and PASS SQLRally Nordic 2013, Stockholm

Into into Locking and Blocking. Dmitri Korotkevitch (http://aboutsqlserver.com) 1

Into into Locking and Blocking Dmitri Korotkevitch (http://aboutsqlserver.com) 1 About me 20 years of experience in IT 14+ years of experience working with Microsoft SQL Server Microsoft SQL Server MVP