balmukund lakhani technical lead – sql support | microsoft gtsc | blakhani@microsoft.com team...
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High Availability options Explored with SQL Server
Balmukund LakhaniTechnical Lead – SQL Support | Microsoft GTSChttp://blogs.msdn.com/blakhani | blakhani@microsoft.comTeam Blog – http://blogs.msdn.com/SQLServerFAQ
About me…
Working with SQL Technology since 2001 (almost 10 years)Currently working as Technical Lead with Microsoft SQL Support team.Premier Field Engineer @ MicrosoftDeveloper @ Ramco SystemsDBA @ Ramco SystemsERP Onsite Support @ Ramco Systems
Agenda
Why High Availability?Backup/restore related technologies.Database SnapshotsLog-shippingDatabase MirroringFailover ClusteringReplicationSQL Server “Denali” High Availability
Business Needs
RTO (Recovery Time Objective)The duration of acceptable application downtime, whether from an unplanned outage or from scheduled maintenance/upgrades
RPO (Recovery Point Objective)The ability to accept potential data loss from an outage
Causes of Downtime and Data Loss
Planned Downtime*Performing database maintenancePerforming batch operationsPerforming an upgrade.
Unplanned DowntimeData center failureServer failureI/O subsystem failureHuman error
What do we need?
Minimize or avoid service downtimeWhether planned or unplanned
When components fail,service interruption is brief or non-existent
Automatic failoverEliminate single points of failure (as affordable)
Redundant componentsFault-tolerant servers
Backup, Restore, and Related Technologies
Partial Database Availability and Online Piecemeal RestoreInstant File InitializationMirrored BackupsBackup ChecksumsDatabase SnapshotsBackup Compression
Minimizing downtime
Backup Size
Backup Time
Restore Time
0
1
2
3
4
5
6
No CompressionCompression
Backup Compression
Backup compressi
on
Hot- Add CPU
Hot-Add memory
Online Operation
s
4.01GB
34.2 MB
295.8s
126.7s219s
126s
What Are Database Snapshots?
Read-only, consistent view of a database
Specified point-in-time
Modifying dataCopy-on-write of affected pages
Reading dataAccesses snapshot if data has changedRedirected to original database otherwise
12:00 Snapshot
Page
Page
Using Database Snapshot to Recover Data
Scenario Example Code / Steps
Undeletingrows
Undoingan update
Recoveringa droppedobject
11 Script the object in the database snapshot
INSERT INTO Production.WorkOrderRoutingSELECT * FROM AdventureWorks_dbsnapshot_1800.Prod.WorkOrderRouting
INSERT INTO Production.WorkOrderRoutingSELECT * FROM AdventureWorks_dbsnapshot_1800.Prod.WorkOrderRouting
UPDATE HR.DepartmentSET Name = ( SELECT Name FROM AdventureWorks_dbsnapshot_1800.HR.Department WHERE DepartmentID = 1)WHERE DepartmentID = 1
UPDATE HR.DepartmentSET Name = ( SELECT Name FROM AdventureWorks_dbsnapshot_1800.HR.Department WHERE DepartmentID = 1)WHERE DepartmentID = 1
Caution: Not a substitute for a comprehensive backup and restore strategy
Execute the script in the source database22
Repopulate the object (if appropriate)33
Transaction log restore - Puzzle
Full backu
p 1 (12:00 PM)
T-Log backu
p 1 (1:00 PM)
T-Log backu
p 2 (2:00 PM)
T-Log backu
p 3 (3:00 PM)
Full backu
p 2 (3:25 PM)
T-Log backu
p 4 (3:35 PM)
03:22 PM
Log Shipping
Log shipping allows you to automatically send transaction log backups from one database to a secondary database on another serverLogs are restored automaticallySecondary server can be the failover server and can become primary if the main server goes down
Copy
Copy
Copy
Perform Backups
Copy and Restore Backups
RaiseAlerts
Primary Database
Monitor Database
Secondary Database
Secondary Database
Secondary Database
Copy and Restore Backups
Copy and Restore Backups
Log Shipping (in Action)
Strength & weakness
StrengthsCan Ship Logs Across WAN (Wide-Area Network)Protects an Entire Database
WeaknessesConfigured Per DatabaseNO AUTOMATIC FAILOVER
Database Mirroring How it works
MirrorPrincipal
Witness
Log
Application
SQL Server SQL Server
2
2
4
51
Data DataLog
3>2 >3
Mirror is always redoing – it remains currentCommit
Database Mirroring ModesHigh-Availability Mode
Safety Full; Synchronous operationDatabase is available whenever a quorum existsAutomatic failover
High-Protection ModeSafety Full; Synchronous operationNo witness – quorum provided by partnersIf Principal loses quorum, it stops servicing the database
Ensures high protection; database is never in ‘exposed’ state
Manual failover only; no automatic failoverA transition mode; should not be in this mode for long
High-Performance ModeSafety Off; Asynchronous operationManual failover only
Supports only one form of role switching: forced service (with possible data loss)
Principal
Witness
Data Log
Mirror
Data Log
1. Bad Page
DetectedX
2. Request page
3. Find page
4. Retrieve page
5. Transfer page6. Write
Page
DBM – Automatic Page Recovery
Client
CPU goes up when compression is on, both because of compression/decompression, but also because the server can now process more transactions per second
Percentage increases in throughput is most dramatic for low network bandwidths
DBM – Log Compression
Strength & Weakness
StrengthsCan Mirror Across WANAutomatic Failover, and Nearly Instantaneous, Better than Failover ClusteringProtects an Entire Database
WeaknessesRequires Enterprise EditionMust be Configured Per Database
Supports many scenarios: • Single Instance• Multiple Instance• Multiple Active Nodes• N+1• N+M
N+1: N Active, 1 Inactive Nodes
Inst2 *
* Inst1
Multiple Active Nodes
Inst2 *
* Inst1Inst3 *
N+M: N Active, M Inactive Nodes
Failover Cluster
* Inst1
SQL Server Cluster Topologies
Failover Clustering (Facts)Redundancy at database instance level
All databases fail over togetherShared copy of system databases
Single data copy on shared storage deviceNo I/O overhead reducing throughputStorage unit is single point of failure for cluster
All database services are clusteredSQL Agent; Analysis Services; Full-Text engine, MS DTC
Automatic failover (up to minutes)DBMS accessed over virtual IPStorage is controlled by one cluster node at a timeRequires hardware certified by Microsoft for Microsoft Cluster Service
Strength & Weakness
StrengthsProvides Protection Against a Node Failure, Protects the Entire SQL InstanceAutomatic Failover Supported
WeaknessesGenerally Expensive, Requires Specialty HardwareSpecialty Hardware RequirementsNot Trivial to Configure and ManageDoesn’t Protect Against a Complete
Site Failure
Replication
Primarily used where availability is required in conjunction with scale out of read activityFailover possible; a custom solutionNot limited to entire database; Can define subset of source database or tablesCopy of database is continuously accessible for read activityLatency between source and copy can be as low as seconds
Peer-to-Peer Replication
Provides high availability and read scalabilityBuilds redundancy by eliminating single point of failureEnable online upgrades of serversMaximize Application UptimeSupport for both Ring and Grid TopologyCentralized Management using Management Studio
Peer Node
Peer Node
Peer Node
Peer Node
New Features
Application Server
Load BalancingRead
Write
Replicated Data
User Request
s
Add and remove nodes without stopping
Visual configuration with Topology Wizard
Ability to detect conflicts
Strength & Weakness
StrengthsPerpetual or on-demand replication of data, local or remoteProtects (duplicates or merges) the exact portion of the database I want
WeaknessesConfigured per database, even per tableGenerally does not protect or duplicate an entire Database
SQL Server AlwaysOn
Increased Application Availability at Lower TCO with Ease of Use:
Multi-Database FailoverMultiple SecondariesActive Secondaries Fast Client Connection RedirectionWindows Server CoreMultisite Clustering
Availability Group Concepts
Availability GroupDefines the high availability requirements
Databases, Replicas, Availability Mode, Failover Mode etc
Availability ReplicaSQL Server Instances that are part of
the availability group which hosts the physical copy of the database
Role: Primary, Secondary, Resolving
Availability DatabaseSQL Server database that is part of an availability groupThis can be a regular database or contained database
Readable Secondary
Readable secondary allow offloading read queries to secondaryClose to real-time data, latency of log synchronization impact data freshness
DB2
DB1
SQLservr.exe
SQLservr.exe
InstanceA
DB2
DB1
Primary Secondary
Database Log Synchronization
InstanceB
Reports
SQL Server “Denali” High Availability
• Configuration• Read-only Secondary
Demo
Comparing High Availability Technologies
CategoryAvailability
FeatureFailover
ClusteringDatabase Mirroring
Log Shipping Replication
Failover characteristics
Standby Type Hot Hot Warm Hot
Automatic role change
Yes Yes No No
Failover TypeAutomatic and
ManualAutomatic and
ManualManual Manual
Database Downtime during
failover
30 secs + database recovery
Less than 10 secs
Variable Variable
Physical Configuration
Redundant Storage
LocationsNo Yes Yes Yes
Hardware requirements
Microsoft certified cluster
solutionsStandard servers Standard servers Standard servers
Physical distance limit
100 miles None None None
Additional server role
None Witness Monitor Distributor
Comparing High Availability Technologies
CategoryAvailability
FeatureFailover
ClusteringDatabase Mirroring
Log Shipping
Replication
Management Complexity High Low Low Medium
Standby Accessible
NoVia Database
SnapshotsRead Only Yes
Multiple Secondaries
No No Yes Yes
Load delay on secondary
No No Yes No
Scope of Availability
Server Instance Database Database Database
Client Access Client redirect None requiredSupport in SNAC
and ADO.NET
Custom coding
required
Custom coding
required
SummarySQL Server High Availability Technology
Database Server Failure or Disaster
Failover ClusteringDatabase MirroringPeer-to-Peer Replication
User or Application ErrorLog ShippingDatabase Snapshot
Data Access Concurrency Limitations
Snapshot IsolationOnline Index OperationsReplication
Database Maintenanceand Operations
Fast RecoveryPartial AvailabilityOnline RestoreMedia ReliabilityDedicated Administration ConnectionDynamic Configuration
Availability at ScaleData PartitioningReplication
TuningDatabase Tuning Advisor
References
SQL Server Code Name Denali CTP1http://www.microsoft.com/sqlserver/en/us/product-info/future-editions.aspx SQL Server Books Online
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Contact
Blog Addresshttp://blogs.msdn.com/BLakhani
http://blogs.msdn.com/SQLServerFAQ
Email Addressblakhani@microsoft.com
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The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after
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