Category: Availability Groups

Ginger Keys shows us two methods for creating Basic Availability Groups in SQL Server 2016 and later:

AlwaysOn Basic Availability Groups (BAGs) are available with SQL Server 2016 and 2017 Standard edition.  The functionality is generally the same as database mirroring (which has been deprecated).  This feature replicates transactions to a database on a secondary server, and is useful for disaster recovery should something happen to the primary server.

If you have a database that requires an extra layer of protection or ‘BAG of tricks’, deploying a Basic Availability Group is useful for providing disaster recovery and high availability for the one database.  Also there is major cost savings since it is not necessary to purchase SQL Enterprise edition…this can be done in Standard edition of SQL Server.

BAGs provide a failover environment for only one database, and there can only be two replicas in the group.  Replication can be synchronous or asynchronous, and there is no read access, no backups and no integrity checks on the secondary.   The secondary replica remains inactive unless there is a failover, and Basic AGs can remain on-premises or span from on-prem to Azure.

Read on for the two methods.

Sourabh Agarwal announces improvements to Availability Groups when it comes to database level failover:

In addition to the existing checks, the new implementation has the following additional checks.

1. The new implementation stores and uses a historical snapshot of the database state information to decide if a failover should be initiated. The health check routine caches the database state and associated error information, for the last two executions, which is then compared with the state information from the current execution of the health detection routine. If the same error condition (for the below mentioned error codes) exists in three consecutive runs of the health detection routine, a failover is initiated. This implementation is intended to provide safeguards against transient errors and issues which can be fixed by the auto page repair capabilities of the availability groups.

2. The new implementation checks for following additional errors. Majority of these errors are indicative of a hardware issues on the server. Please note, that this is not an exhaustive list of errors which could impact the database availability. There is an outstanding item to include error 824 to this list.

Great news from the Tiger Team.

Ryan Adams explains how to create a Read-Scale Availability Group:

A Read-Scale Availability Group is a Clusterless Availability Group.  It’s sole purpose and design is to scale out a read workload.  More importantly is what it is not.  It is NOT a High Availability or Disaster Recovery solution.  Since this design has no cluster under it, you lose things like automatic failover and database level health detection.  For example, You have reports that run for customers that are in your DMZ that is fire-walled off from your internal network.  Opening up ports for Active Directory so that you can have a cluster means opening a ton of ephemeral ports and ports with high attack vectors.  Remember the Slammer worm?  This solution removes those dependencies.

Click through for the setup scripts as well as a video Ryan created of him putting it all together.  As long as you recognize the trade-offs involved, this can be a nice solution to certain problems.

Frank Gill explains one method of building out data in an Availability Group, automatic seeding:

Microsoft released Availability Groups (AG) as a feature in SQL Server 2012. Prior to SQL Server 2016, there were two methods of adding a database to a new AG replica.

1. You could provide the Add Database to Availability Group wizard a file share accessible by the primary and secondary replicas.  SQL Server would run FULL and LOG backups of each database to the share and use them to restore the database(s) to each replica.
2. You could manually run a FULL and LOG backup of each database, copy the backup files to each replica, and restore the databases WITH NORECOVERY.

With SQL Server 2016. Microsoft has provided a third option, Automatic Seeding.  With Automatic Seeding, you specify the databases and the replicas and SQL Server will begin transferring data to each replica.  The duration of the seeding process depends on the size of the database and the network bandwidth available between primary and secondary replica.

Automatic seeding isn’t perfect, but it’s quite useful.

Dong Cao explains the two different models for transaction log redo when using availability groups:

When availability group was initially released with SQL Server 2012, the transaction log redo was handled by a single redo thread for each database in an AG secondary replica. This redo model is also called as serial redo. In SQL Server 2016, the redo model was enhanced with multiple parallel redo worker threads per database to share the redo workload. In addition, each database has a new helper worker thread for handling the dirty page disk flush IO. This new redo model is called parallel redo.

With the new parallel redo model that is the default setting since SQL Server 2016, workloads with highly concurrent small transactions are expected to achieve better redo performance. When the transaction redo operation is CPU intensive, such as when data encryption and/or data compression are enabled, parallel redo has even higher redo throughput (Redone Bytes/sec) compared to serial redo. Moreover, indirect checkpoint allows parallel redo to offload more disk IO (and IO waits for slow disk) to its helper worker thread and frees main redo thread to enumerate more received log records in secondary replica. It further speeds up the redo performance.

Read on to learn more about these two models, including positives and negatives for each and how to switch from one to the other.

Allan Hirt points out two improvements to Availability Groups in the latest SQL Server 2016 and 2017 patches:

First and foremost, SQL Server 2016 Service Pack 2 was just released today. There are two major improvements in it for AGs:

1. SQL Server 2016 now has full Microsoft Distributed Transaction Coordinator (DTC) support. SQL Server 2016 had partial support for DTC with one of the two scenarios (cross instance/cross platform), but not intra-instance DBs. SQL Server 2017 had both, and now that was backported so SQL Server 2016 supports all DTC scenarios with AGs. This is great news.

Click through for the other major improvement.  This is in addition to yesterday’s notice regarding the distribution database.