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Alwayson Availability Groups A Viable Alternative To Failover Clusters?

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Is Alwayson Availability Groups A Viable Alternative To Alwayson Failover Clusters In The Public Cloud?

I recently read an article entitled SQL Server 2012 AlwaysOn: High Availability database for cloud data centers where the author John Joyner makes a case for using AlwaysOn Availability Groups for SQL Server high availability in the cloud. I have been investigating AlwaysOn Availability Groups since it was available in pre-release versions of SQL Server 2012. While it certainly has some valid uses (mostly in disaster recovery configurations), saying that it is a “new way to achieve HA SQL” glosses over many of the issues that make deploying AlwaysOn Availability Groups as a replacement for failover clusters simply not a viable option in many cases.

My Response

In a response I wrote to the article I proposed that an AlwaysOn Multisite Clustering using the host based replication solution DataKeeper Cluster Edition is a much better alternative and I went ahead and explained why..

My original response to the article seems to have been deleted, so I decided to repost my response to the original article below:

There are a few things to consider with AlwaysOn Availability Groups. As you mention, “Microsoft announced support for some System Center 2012 SP1 applications to work with SQL AlwaysOn”, meaning that there are still applications that do not support AlwaysOn. In fact, there are a LOT of applications that do not support AlwaysOn Availability Groups, including any applications that use distributed transactions. And what about the other limitations, like not being able to keep MSDB, Master and other databases in sync? I blog about these limitations here.

https://clusteringformeremortals.com/2012/11/09/how-to-overcome-the-limitations-of-sql-server-alwayson-availability-groups-sqlpass/

I agree that SQL HA is important, however, the only way to get “High Availability” (meaning automatic recovery in the event of a failure) with AlwaysOn Availability Groups is by using synchronous mirroring. At PASS Summitt in Seattle earlier this month I sat in many different presentations on AlwaysOn and almost without fail the presenters talked about AlwaysOn in an asynchronous configuration. The reason being is that AlwaysOn synchronous replication has a SIGNIFICANT impact on the performance of your application. I have personally measured up to a 68% performance penalty with AlwaysOn Synchronous mirroring, and that was across a dedicated 10 Gbps LAN! I blog about this result here

Unfortunately, in an asynchronous configuration you give up automatic failover, so you really are not getting HA, you are getting data protection, but certainly not the same RTO as you can expect from a traditional SQL failover cluster.

And then finally there is the cost to consider. SQL Server 2012 Enterprise is nothing to sneeze at. If you want to build a 2-node cluster and take advantage of readable secondaries and you are using a 2-socket, 16-core servers you are looking at shelling out close to $220k for SQL Server 2012 Enterprise licenses. I broke down the associated cost in my blog article here.

Don’t get me wrong, SQL Server 2012 AlwaysOn Availability Groups can solve many problems, but I would not categorize the asynchronous configuration required in most cloud deployments as an HA alternative. Many people are overlooking the other “AlwaysOn”, AlwaysOn Failover Clusters. New features of SQL Server AlwaysOn Failover Clusters, including enhanced support for cross subnet multisite clusters, will give you a true HA solution and overcomes all of the limitations I describe above. Of course in a pure cloud solution you may not be able to integrate with array based replication to support multisite clusters, but you can always use host based replication solutions such as SteelEye DataKeeper Cluster Edition to build multisite clusters in public or private clouds and in your own physical data center and you can do this with SQL Server 2008 through 2012 AND it works on SQL Server Standard edition as well as Enterprise.

Have you done any testing with AlwaysOn Availability Groups in a HA configuration? If so I’d be curious to know if you measured the overhead associated with synchronous replication in your environment.

Reproduced with permission from https://clusteringformeremortals.com/2012/11/27/is-alwayson-availability-groups-a-viable-alternative-to-alwayson-failover-clusters-in-the-public-cloud/

Sacrifice High Availability For High Performance?

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Mosey on over to the Fusion-io website and read my guest post in their blog, Do You Have to Sacrifice High Availability for High Performance?

Invitation to join our Webinar

After you are done there, view the joint SIOS and Fusion-io webinar “SQL Server 2008 – High Performance and High Availability Through Fusion-io and SIOS”. Part of this webinar includes some VERY interesting benchmark information…you won’t want to miss it!

Reproduced with permission from https://clusteringformeremortals.com/2011/08/23/do-you-have-to-sacrifice-high-availability-for-high-performance/

SQL Server Denali And Hadron

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Microsoft’s latest version of SQL, code named Denali, is currently in CTP. One of the most anticipated feature is something called HADRON. Basically, it is has been described as mirroring 2.0. The best features of HADRON as I can see compared to mirroring as it is in 2005/2008 are as follows:

  • You can query the mirror (now known as a replica)
  • You can have more than one replica
  • Database are grouped into availability groups to ensure related databases always fail over together
  • Support for replicating more than 10 databases

If you want a great preview, check out fellow MVP Brent Ozar’s post, SQL Server Denali Rocks!

From reading Brent’s post I gather Microsoft only plans to include this in the Enterprise version of SQL. That is too bad for the many customers running SQL Server Standard as traditionally there is a significant price jump to go from Standard to Enterprise. However, that is good news for the 3rd party replication vendors out there who are providing similar functionality today on SQL 2005/2008 on Standard and Enterprise Editions.

SteelEye DataKeeper Cluster Edition for SQL Server

While it is not an exact duplicate of HADRON, a multi-site cluster using SteelEye DataKeeper Cluster Edition for SQL Server gives you a great solution for HA/DR without requiring shared storage. So while we wait for Denali and all the application vendors to start supporting Denali you might want to have a look at what SteelEye DataKeeper Cluster Edition can do for you today!

Reproduced with permission from https://clusteringformeremortals.com/2010/12/10/sql-server-denali-and-hadron/

Minimize Downtime Using High Availability Clustering And Data Replication For Sap

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Why implement high-availability cluster configuration?

What does it cost your business when your SAP system is down due to a hardware failure, an administrator mistake or for maintenance activities?  The impact can range from thousands of dollars lost in sales per minute to significant drops in employee productivity and customer satisfaction.  Through the implementation of a high-availability cluster configuration, downtime from both planned and unplanned outages can be virtually eliminated.

SteelEye and SAP AG have worked closely together to ensure that the availability needs which corporate users have for critical business infrastructure running on Linux and Windows platforms are achieved through the deployment of monitoring and switchover solutions for SAP Application Server (AS) and both JAVA and ABAP Web Application Server (WAS) environments.

Please join Steel Eye EVP of Product Management, Bob Williamson as he explains how you can minimize your downtime using SAP High Availability with SteelEye.

Reproduced with permission from https://clusteringformeremortals.com/2010/06/30/minimize-downtime-using-high-availability-clustering-and-data-replication-for-sap/

 

Will Windows Failover Clusters Become Obsolete?

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Will Windows Failover Clusters Become Obsolete?

I was recently asked whether Windows Failover Clusters (MSCS/WSFC) will become obsolete due to 3rd party HA solutions. To me, there will always be a market for 3rd party HA solutions. But many of the enhancements delivered with Windows Server 2008 have reduced the need to explore alternate HA solutions.  I think the greater threat to MSCS/WSFC is HA solutions provided by the virtualization vendors. This includes Microsoft’s Hyper-V failover clusters (which actually uses WSFC) and VMware HA.  These solutions provided by the virtualization platform provide protection in case of host failure. Although, they currently do not have visibility into the application that is running within the VM.

The real question is what kind of failure do you want to protect against? If physical server failure is your primary concern, then in some cases where MSCS may have previously been deployed, you will see Hyper-V Clusters or VMware HA being deployed instead.  In other cases where MSCS/WSFC may have seemed like overkill or was incompatible with the OS or application, you will instead see clustered VMs being deployed because it is easy to install and it supports all applications and operating systems. The mere fact that more workloads will be running per physical server will make it imperative to have some kind of clustering solution so that the failure of a single server does not bring down your entire infrastructure. In many cases, this clustering solution will be provided by the virtualization vendor.

Hyper-V Clusters and VMware HA are easy to implement and have a broad range of support as the protected VM can be running any OS or application.  The tradeoff is that you lose the application level monitoring included with MSCS/WSFC.  There will always be a class of applications that need application awareness. So MSCS/WSFC or other HA solutions that manage application availability will always be needed to ensure that the application is available, not just the server itself. With that being said, MSCS/WSFC will not become obsolete. Instead, you will see it deployed alongside other cluster solutions provided by the hypervisor vendors.

Read more about Windows Failover Clusters

Reproduced with permission from https://clusteringformeremortals.com/2010/03/01/will-windows-failover-clusters-mscswsfc-become-obsolete/