Why Availability Groups?- It is the hottest topic in SQL Server 2012.

Since it is a new topic, Many database administrators and system administrators will have a lot of questions and hence will search the web for information. So I thought, why not aggregate the information and put it in one place.

I will try to keep the list as updated as possible.

So here is the List.

Prerequisites, Restrictions, and Recommendations for AlwaysOn Availability Groups (SQL Server)

https://msdn.microsoft.com/en-us/library/ff878487(v=SQL.110).aspx#RestrictionsAG

General Info about Availability groups 

A series of Blog posts about AlwaysOn availability groups from BrentOzar PLF

 https://www.brentozar.com/sql/sql-server-alwayson-availability-groups/ 

https://www.simple-talk.com/sql/database-administration/sql-server-2012-alwayson/

Configuring AlwaysOn with Powershell

https://blogs.msdn.com/b/sqlalwayson/archive/2012/02/03/configuring-alwayson-with-sql-server-powershell.aspx

Availability Group Listener

Video about AlwaysOn Availability Group Listener.

https://blogs.msdn.com/b/sqlalwayson/archive/2012/01/16/introduction-to-the-availability-group-listener.aspx

Creating Multiple listeners for the same availability group

https://blogs.msdn.com/b/sqlalwayson/archive/2012/02/03/how-to-create-multiple-listeners-for-same-availability-group-goden-yao.aspx

Sp_Server_diagnostics

https://blogs.msdn.com/b/psssql/archive/2012/03/08/sql-server-2012-true-black-box-recorder.aspx

Flexible failover policy

https://msdn.microsoft.com/en-us/library/ff878664(v=SQL.110).aspx

Readable Secondary

This Blog post by Remus Rusanu about Availability Groups is simple and easy to understand.

https://rusanu.com/2010/11/11/alwayson-high-availability-and-reads-scale-out/

The AlwaysOn Blog has excellent posts about Readable Secondaries. All the Internals that you want to learn about Active Secondaries
can be found here.(Temporary Statistics, Snapshot Isolation etc...)

https://blogs.msdn.com/b/sqlalwayson/archive/tags/alwayson_3a00_+readable+secondary/

Configuring Secondary Read Only Access

https://www.bidn.com/blogs/PatrickLeBlanc/ssis/2513/alwayson-configuring-secondary-read-only-access

Readable Routing

Blog post about Configuring Readable Secondary routing

https://blogs.msdn.com/b/jtarquino/archive/2011/12/12/configuring-a-readable-secondary-routing-in-sql-server-2012-alwayson.aspx

Wiki Article on Read only Routing

https://social.technet.microsoft.com/wiki/contents/articles/13503.read-only-routing-with-sql-server-2012-always-on-database-availability-groups-en-us.aspx?Sort=MostUseful&PageIndex=1

A Very handy script that will generate the ROR URL for you-  Written By matt Neerincx

https://blogs.msdn.com/b/mattn/archive/2012/04/25/calculating-read-only-routing-url-for-alwayson.aspx

AlwaysOn and Backups

An excellent 3-part series by fellow PFE ,Lisa Gardner on Backups on  Active Secondaries.

https://blogs.msdn.com/b/sqlgardner/archive/2012/07/18/sql-2012-alwayson-and-backups-part-1-offloading-the-work-to-a-replica.aspx

https://blogs.msdn.com/b/sqlgardner/archive/2012/07/21/sql-2012-alwayson-and-backups-part-2-configuring-backup-preferences-and-automating-backups.aspx

https://blogs.msdn.com/b/sqlgardner/archive/2012/08/28/sql-2012-alwayson-and-backups-part-3-restore.aspx

Availability Groups and Kerberos

A blog post by Robert about Using Availability Groups with Kerberos

https://blogs.technet.com/b/rhartskeerl/archive/2012/03/04/kerberos-and-availabilitygroups-what-you-need-to-know.aspx

Asymmetric Storage

Excellent Post by Pareekshit explaining about Asymmetric Storage.

https://www.sqlserverfaq.net/2012/09/16/when-do-we-need-asymmetric-storage-with-alwayson-fci-and-availability-storage/

Converting a Logshipping configuration to availability group

Blog post explaining ,how you can convert your existing Logshipping architecture to an Availability Group Architecture.

https://blogs.msdn.com/b/sqlalwayson/archive/2012/01/09/converting-a-logshipping-configuration-to-availability-group.aspx

Troubleshooting and Monitoring

Joe Sack’s blog post about the various wait statistics associated with Availability groups

https://www.simple-talk.com/sql/database-administration/a-first-look-at-sql-server-2012-availability-group-wait-statistics/

Another blog post by Joe Sack about  Always on Dashboard and the DMV’s behind the dashboard.

https://www.sqlskills.com/blogs/joe/post/Answering-Questions-with-the-AlwaysOn-Dashboard.aspx

Information about the  AlwaysOn Extended Event session –written by Jonathan Kehayias

https://www.sqlskills.com/blogs/jonathan/post/New-AlwaysOn_health-Extended-Events-Session-in-SQL-Server-2012-RC0.aspx

Monitoring with Powershell 

https://blogs.msdn.com/b/sqlalwayson/archive/tags/powershell/

Internals

Various Blog posts by the Customer Service and Support Team about the internals of AlwaysOn

https://blogs.msdn.com/b/psssql/archive/tags/alwayson/

FAQ

Allan Hirt  has an Excellent post on various questions on Availability Groups

https://www.sqlha.com/2012/04/13/allans-alwayson-availability-groups-faq/

My fellow PFE and friend Krishna has an Excellent one line answers for various Questions regarding AG

https://blogs.msdn.com/b/srgolla/archive/2012/09/17/sql-server-2012-always-on-faqs.aspx

Another FAQ blog post about AlwaysOn

https://blogs.technet.com/b/sqlman/archive/2011/08/14/sql-server-denali-alwayson-q-amp-a.aspx

White Papers

AlwaysOn Architecture Guides and Design Patterns

These Guides explain you about the various design patterns you can implement with AlwaysOn

•       
  HA and DR using Failover Cluster Instances.(Multi Subnet)
•       
  HA and DR using using Availability Groups.
•   
  HA using FCI and DR using Availability Groups

https://blogs.msdn.com/b/sqlalwayson/archive/2012/07/03/alwayson-architecture-guides.aspx

Migration Guide:
Migrating to SQL Server 2012 Failover Clustering and Availability Groups from Prior Clustering and Mirroring Deployments

https://msdn.microsoft.com/en-us/library/hh923056.aspx

When,troubleshooting any issue related to SQL Server, one of the first things we ask the customer  is the SQL Server ErrorLog. Many times, we get the question, where is it located?

There  are several ways to determine the path of the ErrorLog.

 My fellow Microsoft colleague Balmukund has a blog post explaining the various ways to find the path of the ErrorLog. 

https://sqlserver-help.com/2011/06/26/help-where-is-sql-server-errorlog/

Apart from the above described methods, One other way, that we could get the path of the ErrorLog  in SQL Server 2012,when the SQL Server service is running  is through this DMV. 

Sys.dm_os_server_diagnostics_log_configurations

SELECT is_enabled,[path],max_size,max_files

FROM

Sys.dm_os_server_diagnostics_log_configurations

Executing this DMV will give valuable information about the Errorlog  like

•  Path indicating the location of the diagnostic logs.
•  Maximum size in megabytes to which each of the diagnostic logs can grow.
•  Maximum number of diagnostic log files that can be stored on the computer before they are recycled for new diagnostic logs.
•  If logging is enabled or not.

Hope this helps!!

Through this blog post, I just wanted to share a troubleshooting scenario that I had with one of my customers.

Issue: Having some sporadic issues with reports coming from our SharePoint 2010/Reporting server.

They were seeing the following error and this was happening for only a few reports.

So we had a meeting convened to discuss with the Customer team about the issue and to further troubleshoot it.

The Q& A started:

Q  When did this issue start?

It has been around 2 days.

Q  Were any Changes made to any of the components involved in generating the Reports?

No changes were made, but the server on which the ReportServerDB is hosted, had a Disk failure and had to recover all the databases from previous backups.

 Q Are the reports which are failing getting back a lot of data?

 No, there are reports that get a lot of data, but even the smaller ones are failing.

Then we asked for the Error logs on the SQL Server instance. In the mean time I asked them to Query the Executionlog2 View in the Report Server Database to check the retrieval time, processing time & rendering time of the Reports that ran successfully.

So we ran the following query

select * from ExecutionLog2 order by TimeStart DESC

The query did not complete and gave us the below error.

Msg 601, Level 12, State 3, Line 2

Could not continue scan with NOLOCK due to data movement.

Then I did a quick search on the error and found the following about the above error.

“When there is corruption in database (or) When scanning the data with the NOLOCK locking hint (or) with the transaction isolation level set to READ UNCOMMITTED, it is possible for the page at the current position of the scan would have deleted or moved by page splits caused by Inserts/Updates/Deletes making SQL Server not able to scan further and cause Error 601: Could not continue scan with NOLOCK due to data movement.”

 Since we did not run under READ UNCOMMITTED and No locking hint was provided, the suspicion was that the database could be corrupted. This was also strengthened by the fact that they had disk failure earlier.

 To confirm my suspicions, we executed DBCC CHECKDB on the database. But the CHECKDB was progressing very slowly. When we checked the Estimated Completion Time , it was giving us 25 hours. So we killed the CHECKDB session.

By this time, we got the error logs from the SQL Server instance that was hosting this Database and parsing through the error logs found multiple occurrences of the following error.

Error: 605, Severity: 21, State: 3. ... Attempt to fetch logical page (1:3360) in database 6 failed. It belongs to allocation unit 0 not to 72057594040811520.

Database id 6 happens to be the Report Server database that is being used for reporting purposes in the SharePoint-Integrated mode.

Error 605 occurs when SQL Server detects database corruption.

When the severity is Level 21, the user session is disconnected and the error is written into the SQL ERRORLOG and Windows Application Event Log as Event ID=605

Though this confirms our original suspicion, the DBCC CHECDB was not completing and we wanted further proof through the DBCC Checks.

Since we know the allocation unit id from the error log, (72057594040811520) the plan was to find the corresponding object and run DBCC CHECKTABLE on that object.

So we ran this query in the context of the report server database.

SELECT au.allocation_unit_id, OBJECT_NAME(p.object_id) AS table_name, fg.name AS filegroup_name,
 
 au.type_desc AS allocation_type, au.data_pages, partition_number
 
 FROM sys.allocation_units AS au
 
 JOIN sys.partitions AS p ON au.container_id = p.partition_id
 
 JOIN sys.filegroups AS fg ON fg.data_space_id = au.data_space_id
 
 WHERE au.allocation_unit_id = '72057594040811520'
 
 ORDER BY au.allocation_unit_id

 The result was the CATALOG table.

 We executed DBCC CHECKTABLE on the table and this completed giving the following results

There are 224 rows in 31 pages for object "Catalog".

CHECKTABLE found 0 allocation errors and 695 consistency errors in table 'Catalog'

The issue was that the backup from which this database was restored also got corrupted. The issue was resolved by restoring from a previous non corrupted backup.

Once they did that, the reports also started to function normally.

 Reference: https://support.microsoft.com/kb/2015739