Enhancements in SQL Server 2016 Availability Groups

SQL Server 2016 is making some significant improvements to the Always On Availability Groups set of features. There are a number of features, such as:

1. Round-robin load balancing in readable secondaries

2. Increased number of auto-failover targets                

3. Enhanced log replication throughput and redo speed

4. Support for group-managed service accounts           

5. Support for Distributed Transactions (DTC)               

6. Basic HA in Standard edition                                     

7. Direct seeding of new database replicas                    

Automatic Seeding in Always On Availability Groups

Automatic Seeding is a feature introduced with SQL Server 2016. We need to add SEEDING_MODE = AUTOMATIC when creating the Availability Group or  alter the existing availability group as follows. If you set this feature AUTOMATIC with the following script, each database automatically added to the secondary server by SQL Server.

Enable Automatic Seeding

In order to do this, there should be paths of the same name for the data and log files on the primary and secondary servers.

Disable Automatic Seeding

We can set it MANUAL with the help of the script below.

ALTER AVAILABILITY GROUP [AG_Name]

 MODIFY REPLICA ON 'secondary_instance'

 WITH (SEEDING_MODE = MANUAL)

Check Automatic Seeding Status of an Availability Group

You can query the status of automatic seeding with the help of the following queries.

SELECT * FROM sys.dm_hadr_automatic_seeding

 SELECT * FROM sys.dm_hadr_physical_seeding_stats;

SQL Server Always On  is one of the interesting feature introduced with SQL Server 2012 version. I am listing the Questions with answers which are asked on Always On Feature in the SQL Server DBA interviews.

What is the query to check ENDPOINT?

SELECT type_desc, port FROM sys.TCP_endpoints

17. What are Readable Secondary Replicas?

The AlwaysOn Availability Groups active secondary capabilities include support for read-only access to one or more secondary replicas (readable secondary replicas). A readable secondary replica allows read-only access to all its secondary databases. However, readable secondary databases are not set to read-only. They are dynamic. A given secondary database changes as changes on the corresponding primary database are applied to the secondary database.

18. What are the benefits of Readable Secondary Replicas?

Directing read-only connections to readable secondary replicas provides the following benefits:

• Offloads your secondary read-only workloads from your primary replica, which conserves its resources for your mission critical workloads. If you have mission critical read-workload or the workload that cannot tolerate latency, you should run it on the primary.

• Improves your return on investment for the systems that host readable secondary replicas.

In addition, readable secondaries provide robust support for read-only operations, as follows:

• Temporary statistics on readable secondary database optimize read-only queries. For more information, see Statistics for Read-Only Access Databases, later in this topic.

• Read-only workloads use row versioning to remove blocking contention on the secondary databases. All queries that run against the secondary databases are automatically mapped to snapshot isolation transaction level, even when other transaction isolation levels are explicitly set. Also, all locking hints are ignored. This eliminates reader/writer contention.

19. How many synchronous secondary replicas can I have?

We can have up to 2 synchronous replicas, but we are not required to use any. We could run all Secondaries in Async mode if desired

20. Can we use a secondary for reporting purpose?

Yes. An active secondary can be used to offload read-only queries from the primary to a secondary instance in the availability group.

21. Can we use secondary replicas to take the db backups?

Yes. An active secondary can be used for some types of backups

22. What all types of DB backups are possible on Secondary Replicas?

• BACKUP DATABASE supports only copy-only full backups of databases, files, or filegroups when it is executed on secondary replicas. Note that copy-only backups do not impact the log chain or clear the differential bitmap.

• Differential backups are not supported on secondary replicas.

23. Can we take Transaction log backups on the secondary replicas?

Yes, we can take transaction log backups on the secondary replicas without COPY_ONLY option.

 List out some of the requirements to setup a SQL Server failover cluster.

Virtual network name for the SQL Server, Virtual IP address for SQL Server, IP addresses for the Public Network and Private Network(also referred as Heartbeat) for each node in the failover cluster, shared drives for SQL Server Data and Log files, Quorum Disk, and MSDTC Disk.

24. What is “Failover” in Always ON?

Within the context of a session between the primary replica and a secondary replica, the primary and secondary roles are potentially interchangeable in a process known as failover. During a failover the target secondary replica transitions to the primary role, becoming the new primary replica. The new primary replica brings its databases online as the primary databases, and client applications can connect to them. When the former primary replica is available, it transitions to the secondary role, becoming a secondary replica. The former primary databases become secondary databases and data synchronization resumes.

25. How many types of Failover are supported by Always ON?

Three forms of failover exist—automatic, manual, and forced (with possible data loss). The form or forms of failover supported by a given secondary replica depends on its availability mode,

26. What are the Failover types supported by Synchronous-commit mode?

• Planned manual failover (without data loss)

• Automatic failover (without data loss)

27. What is planned manual failover?

A manual failover occurs after a database administrator issues a failover command and causes a synchronized secondary replica to transition to the primary role (with guaranteed data protection) and the primary replica to transition to the secondary role. A manual failover requires that both the primary replica and the target secondary replica are running under synchronous-commit mode, and the secondary replica must already be synchronized.

28. What is Automatic failover?

An automatic failover occurs in response to a failure that causes a synchronized secondary replica to transition to the primary role (with guaranteed data protection). When the former primary replica becomes available, it transitions to the secondary role. Automatic failover requires that both the primary replica and the target secondary replica are running under synchronous-commit mode with the failover mode set to “Automatic”. In addition, the secondary replica must already be synchronized, have WSFC quorum, and meet the conditions specified by the flexible failover policy of the availability group.

29. Can we configure Automatic failover of Availability Groups with SQL Server Failover cluster instances?

SQL Server Failover Cluster Instances (FCIs) do not support automatic failover by availability groups, so any availability replica that is hosted by an FCI can only be configured for manual failover.

30. What are the Failover types supported by under asynchronous-commit mode?

Only form of failover is forced manual failover (with possible data loss), typically called forced failover. Forced failover is considered a form of manual failover because it can only be initiated manually. Forced failover is a disaster recovery option. It is the only form of failover that is possible when the target secondary replica is not synchronized with the primary replica.

31. What is Use the AlwaysOn Dashboard

Database administrators use the AlwaysOn Dashboard to obtains an at-a-glance view the health of an AlwaysOn availability group and its availability replicas and databases in SQL Server 2012. Some of the typical uses for the AlwaysOn Dashboard are:

• Choosing a replica for a manual failover.

• Estimating data loss if you force failover.

• Evaluating data-synchronization performance.

• Evaluating the performance impact of a synchronous-commit secondary replica

• Replica roll-up state

• Synchronization mode and state

• Time to restore log

Question: Does AG support automatic page repair for protection against any page corruption happens?

Answer: Yes, Automatic page repair is supported by Always On Availability Groups. After certain types of errors corrupt a page, making it unreadable, an availability replica (primary or secondary) attempts to automatically recover the page. The partner/replica that cannot read the page requests a fresh copy of the page from its partner or from another replica. If this request succeeds, the unreadable page is replaced by the readable copy, and this usually resolves the error. Detailed documentation.

Page Types That Cannot Be Automatically Repaired

Automatic page repair cannot repair the following control page types:

• • File header page (page ID 0).

  • Page 9 (the database boot page).

  • Allocation pages: Global Allocation Map (GAM) pages, Shared Global Allocation Map (SGAM) pages, and Page Free Space (PFS) pages.

Quorum Status

For cluster type WSFC, displays the quorum status for the availability replica, one of:

 In here you will be choosing the replicas that you want to add to your group. Important features here:

1. Initial Role: You will set the role that each replica will have once that the group becomes available

2. Failover Mode: Manual or Automatic

3. Availability Mode: Synchronous commit or Asynchronous commit

4. Readable Secondary: No, Yes - Read Intent Only and Yes. If you choose No or Yes - Read Intent Only, you won't be available to query your replicas, the Read Intent is when you enable that feature that routes the read operations to your available replicas. With the Yes option you will be able to query the databases in your replica.

Read-intent only -->Only read-only connections are allowed to secondary databases of this replica. The secondary database(s) are all available for read access. 

5. The endpoints tab will show you the URL and ports each replica is set to.

6. Backup preferences tab: in here you can configure if you want to take the backups from the primary or the secondary servers.

7. Listener tab: In this window it lets you set the parameters to configure the availability group listener, however my advice is to do it later, configure your group first and once that is done, configure it later.

8. Read-Only Routing: tab This lets you configure your read only routing for the read-intent setup, this allows you to load balance the queries so you have only the queries that will Insert, Delete or Update on the primary and all the Select queries routed to your secondary's so they don't consume resources the Primary will use (I'll explain it in another post) 

What would happen if you add a new file (database file MDF or transaction log file LDF) to a database which is part of a high availability solution

USE [master]

GO

ALTER DATABASE [MyConDB]

ADD FILE (NAME = N'SQLAuthority_2',

FILENAME = N'E:\SQLAuthority.ndf' ,

SIZE = 4096KB, FILEGROWTH = 1024KB) TO FILEGROUP [PRIMARY]

GO 

Whatever modification operation done on the database would be logged to the transaction log file, called as log record. Since database is part of availability group the same command would be sent to all secondary replicas via log record. This means that adding data or log file to primary database essentially roll-forwards the same command to all secondary replicas. 

Verify that a connection is to a read-only replica

SELECT DATABASEPROPERTYEX(DB_NAME(), 'Updateability');