SQL Server AlwaysOn
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Welcome
Welcome
Speaker: John Ecken
Topic: SQL Server AlwaysOn Technology
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Introducing SQL Server 2014
DR
Introducing SQL Server 2014 AlwaysOn
• Fail-over Clustering
(Shared Drive)
• Database Mirroring
• Always On
• Log Shipping
• Replication
Improvements with
Always On
Improved Efficiency and Cost-Effectiveness
Efficiency and cost-effectiveness are improved through the
implementation of:
– AlwaysOn Readable Secondaries
– Contained Databases
– Multi subnet availability
What happens when…
Business requirements are for automatic failover of
multiple databases together with no single point of
failure and redundancy across multiple datacenters
• In SQL Server 2008 R2 or prior
• Database mirroring provides automated failover of a single database
• Redundant copies of the database exist on principle and mirror server
• Failing over multiple databases at the same time requires custom code logic to
detect single failure and then initiate failover of other databases
• Application failover is accomplished through the use of FailoverPartner
connection string value
• In SQL Server 2012
• Redundant copies (up to four) of the databases exist on nodes participating in
the Availability Group, maintained either synchronously or asynchronously
• Multiple-database failover is handled automatically by the Availability Group
• Application failover through the Availability Group Listener and Application Virtual
Name
Groups
AlwaysOn Availability Groups
Enhance the capabilities of database mirroring
– Multiple database coordinated failover for applications that require multiple
databases on a single instance (e.g. SharePoint)
– Simplified application connectivity and automatic redirection through the
implementation of Availability Group Listener and Application Virtual Name
– Built in compression and encryption
– Synchronous or asynchronous data movement
– Automatic or manual failover modes with configurable failover trigger levels
– Automatic repair of page corruptions
– Readable secondary replicas
– Support for FILESTREAM, FILETABLE, RBS and Service Broker
– Simplified configuration wizards, PowerShell integration and Availability Group
Dashboard for monitoring
Topology examples
A
A
A
A
Direct attached storage local, regional and geo secondaries
Synchronous
data movement
Asynchronous
data movement
What happens when…
You need to move a database to a different SQL Server (Containment)
• In SQL Server 2008 R2 or prior
• SQL Server Logins are mapped to Database Users through login SIDs
• Incorrectly mapped logins result from creating logins on additional servers
• Can result in problems during failovers or when restoring databases to new
servers
• Requires manual transfer of login using sp_help_revlogin or SSIS Transfer
Logins task to maintain identical SID on both servers
• Nothing tracks external dependencies that might exist in the database
code
• In SQL Server 2012
• Partially Contained Databases contain Database Login Credentials as a part of
the database
• Simplifies failover planning for the environment by allowing the login
information to be persisted as a part of the database
• Developers can track instance level impacts and uncontained dependencies
with
Containment Demo
AlwaysOn in SQL Server 2014
What’s New
• What’s being delivered
•
•
•
•
Increase number of secondaries from four to eight
Increase availability of readable secondaries
Support for Windows Server 2012 CSV
Enhanced diagnostics
• Main benefits
• Further scale out read workloads across (possibly geo-distributed) replicas
• Use readable secondaries despite network failures (important in geo-distributed
environments)
• Improve SAN storage utilization
• Avoid drive letter limitation (max 24 drives) via CSV paths
• Increase resiliency of storage failover
• Ease troubleshooting
Groups in 2014
Increase Number of Availability Group Secondaries
Description
•
•
Increase number of secondaries (4–8)
Max number of sync secondaries is still two
Reason
•
Customers want to use readable secondaries
• One technology to configure and manage
• Many times faster than replication
•
Customers are asking for more database
replicas (4–8)
• To reduce query latency (large-scale
environments)
• To scale out read workloads
Always On Group Demo
Readable Secondary
Client Connectivity
Client connection behavior determined by the Availability
Group Replica option
– Replica option determines whether a replica is enabled for read access when in a
secondary role and which clients can connect to it
– Choices are:
• No connections
• Only connections specifying Application Intent=ReadOnly connection property
• All connections
Read-only Routing enables redirection of client connection to
new readable secondary after a failover
– Connection specifies the Availability Group Listener Virtual Name plus Application
Intent=ReadOnly in the connection string
– Possible for connections to go to different readable secondaries if available to
balance read-only access
Readable secondary
Client connects to the Availability
Group Listener virtual name
– Standard connections are routed to
the Primary server for read/write
operations
– ReadOnly connections are routed to a
readable secondary based on
ReadOnly routing configuration
Availability
Group Listener
ReadOnly Routing
Readable secondary
Query Performance on the Secondary
Challenges:
– Query workloads typically require index/column statistics so the query optimizer can
formulate an efficient query plan
– Read-only workloads on a secondary replica may require different statistics than the
workload on the primary replica
– Users cannot create different statistics themselves (secondaries can’t be modified)
Solution:
– SQL Server will automatically create required statistics, but store them as temporary
statistics in tempdb on the secondary node
If different indexes are required by the secondary workload, these must be created
on the primary replica so they will be present on the secondaries
– Care should be taken when creating additional indexes that maintenance overhead
does not affect the workload performance on the primary replica
Readable secondary
Offloading Backups to a Secondary
Backups can be done on any replica of a database to offload I/O from
primary replica
– Transaction log backups, plus COPY_ONLY full backups
Backup jobs can be configured on all replicas and preferences set so that a
job only runs on the preferred replica at that time
– This means no script/job changes are required after a failover
Transaction log backups done on all replicas form a single log chain
Database Recovery Advisor tool helps with restoring backups from multiple
Secondaries
Readable secondary
Workload impact on the secondary
Read-only workloads on mirror database using traditional database mirroring can
block replay of transactions from the principal
Using Readable Secondaries, the reporting workload uses snapshot isolation to
avoid blocking the replay of transactions
– Snapshot isolation avoids read locks which could block the REDO background
thread
– The REDO thread will never be chosen as the deadlock victim, if a deadlock
occurs
Replaying DDL operations on the secondary may be blocked by schema locks
held by long running or complex queries
– XEvent fires which allows programmatic termination/resumption of reporting
Readable Secondary
Demo
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