Recently a group of developers were looking for high availability solutions with SQL Server and ran into some problems because they believed some common myths regarding SQL Server and clustering. In this tip we'll look at some of these myths and correct the common misconceptions.
Myth #1: SQL Server Clustering Provides Better Performance Because each Node Handles Part of the Load
The confusion arises here because SQL Server is installed on each node in the cluster. Adding to this confusion is the additional capability for SQL Server to have multiple instances of SQL Server running in a cluster. This type of installation is typically called an Active-Active cluster meaning each node has an active installation of SQL Server running. However, the SQL Server services which run on each node, each run separate databases, in separate file systems; the data and file systems are never shared. A better name which Microsoft prefers is a multi-instance installation.
Take a look at example 1 below for a single instance of SQL Server. In this illustration the Green node is the installation of SQL Server that is running the SQL1 instance. When you look in the Control Panel | Services application you can see that SQL Server is running. On the gray node an instance of SQL is installed, but is shut down. In the Control Panel | Services application you see the SQL Server service is not running. An instance of SQL Server is installed on each node that acts as part of the clustered instance, but SQL Server runs on only one node at a time. If the instance of SQL Server were to be shut down and moved to another node, then the service would be shut down on the original node and then started on the new node running SQL Server.
In example 2, often referred to as an Active-Active Cluster, there are three nodes with two instances of SQL Server running. Accounting is running on the first node and HR is running on the second. The dotted line indicates that the other node doesn't have access to the database instance, but could take over if needed. Notice the third server, this server has SQL Server installed and could take over as well. SQL Server is installed, but is not running at this time.
Myth #2: There is no SQL Server Down Time if the SQL Server Instance fails from one Node to the other
As the first myth explained, SQL Server is running on one node and is not started on the other node(s). When a failover condition occurs, SQL Server needs to startup on the new node that is taking over the instance of SQL Server. Whether the instance has failed or is restarted intentionally the effect is the same. SQL Server is starting up on the new node and proceeds through the recovery process which is the same process for SQL Server whether it is on a cluster or a non-clustered instance.
Since SQL Server may take anywhere from a few moments to several minutes to start, the virtual name of SQL Server and the instance of SQL Server are not available until the server recovers and the instance starts accepting connections. In addition, any transactions that have not been committed at the time of the failover are rolled back just as they are with a non-clustered instance of SQL Server.
We also need to recognize that when SQL Server restarts applications need to handle the issue of how to reconnect to the instance of SQL Server. SQL Server has no mechanism for storing in-flight transactions and connections. From the application point of view, SQL Server has either disappeared or been restarted. The application vendor will need to determine how and what to do when this occurs.
Myth #3: The SQL Server AlwaysOn Availability Groups feature in SQL Server 2012 requires that SQL Server be Clustered
SQL Server availability groups technology is new to SQL Server 2012 and has some resemblance to SQL Server mirroring technology. Availability groups are a grouping of databases with a virtual name and IP address that act as a single unit for users to access. If the server or file systems fail with an availability group on them, then the entire group is failed over to another replica on another server.
The confusion about SQL Server in a cluster with Availability Groups is pretty easy to understand. To deploy SQL Availability Groups SQL Server must be installed on a server that is part of a Windows Cluster; SQL Server itself, however, does not need to be clustered. What's more, Microsoft has taken the unfortunate position of also calling SQL Server Clustering, "Always On", which leads to confusion that clustering and availability groups are the same thing.
Example 3 shows failover cluster manager for our server. The availability group, ag_accounting, is listed as being online, but notice SQL Server is not present in the list of "Services and Applications" because it is not clustered.
In the next example, example 4, we see how an Availability Group looks through SQL Server Management Studio. You'll first notice the databases indicate they are synchronized. In our example databases AccountsDatabase, AcctsPayableDatabase, and AccountsReceivable are all part of an Availability Group and would fail over together. The other two databases, Finance and FinComm are not part of an Availability Group and act as a database on a typical server. Inside the Availability Groups node in SQL Server Management Studio we see the databases listed in the Availability Databases node.
Myth 4: Other SQL Server Clustered Nodes can be used for Reporting Purposes
SQL Server 2012 does offer an AlwaysOn Availability option which allows the data from SQL Server to be accessed from another server, but SQL Server Clustering does not. SQL Server Clustering contains one set of databases that exist on a shared storage array such as the EMC DMX family of storage. An instance of SQL Server is installed on one more nodes, which can access the data. The access is restricted to one SQL Server instance unless a failover occurs or the SQL Server Cluster is manually failed over to another node.
The option which does allow read only access to other nodes is the SQL Server AlwaysOn Availability Groups feature and the implementation is moderately more involved than setting up a single clustered instance. SQL Server Availability Groups work by copying the data in real time or near real time to another instance of SQL Server and then that data can be made available as a read only replica. With SQL Server AlwaysOn Availability Groups the data can be applied at the destination server synchronously or asynchronously insuring zero latency or minimal latency.
Myth 5: Non Critical SQL Server Databases on a Clustered Instance can reside on Disks that are not Clustered to Save Money
SQL Server 2012 and all previous versions of SQL Server require databases be created on clustered resources. Internal drives or drives which are not part of the cluster group cannot hold user databases. To create a database the syntax remains the same, but SQL Server will perform an additional check to verify the resource is clustered. Also note that SQL Server will accept ANY drive path making no distinction between different levels of RAID technology such as RAID 5 or RAID 1. It is up to the DBA to ensure drives can handle the load which will be placed on them.
In the following illustration, Example 5, the output from a CREATE DATABASE T-SQL statement is issued after attempting to place a database on non-clustered disk.
Myths 1 thru 4 would seem to come from someone who is relatively new to SQL*Server and clustering but somewhat more familiar with Oracle. Your SQL*Server cluster is a shared nothing install meaning that the datafiles for the clustered database are available to one node or the other, but not both at the same time. Whereas Oracle's RAC is a shared everything wher there are 2 or more instances of the database runnign on seperate nodes but accessing the same datafiles and as such behaves as the "myth" suggests. You could call it a weakness in SQL*Server or maybe a weakness in Oracle sicne in SQL only one node can make a mistake whereas in Oracle any node can blow the database to heck.
Thursday, December 13, 2012 - 9:57:53 AM - Earl Cooper
Myth #1 concerns what we now call multi-instance clustering (formerly Active/Active) but Example 1 and its diagram are nothing to do with the myth. I found that confusing.
In the same myth Example 2 illustrates how each node in the cluster can handle part of the load, as long as the load is split over two databases. I think you need to clarify that the myth applies only to the load of users in one database.
Perhaps you should have a diagram of what the myth is and what the reality is, and take out the single-instance diagram.
@Earl -- Availability groups with automatic failover do not make use of a witness server-- Microsoft cluster services manages that. SQL Server itself does ~not~ need to be clustered but to make use of Availability groups the instance of SQL Server must be installed on a server that is part of a cluster.
@Richard -- Sorry for the confusion; as you mentioned Microsoft Cluster services for SQL Server do not offer a shared storage option. One node, and only one node, may access the instance at a time. I'll try to improve my Visio' in the future.
@Mark -- In example 2 we're trying to show that separate instances of SQL can be installed in a cluster, but, as our example demonstrates the HR and Acct. databases do not share data. They are separate installations and do not interact with one another.
Thursday, December 13, 2012 - 11:50:25 AM - Nagesh
One note worth mentioning for Myth #5 is SQL 2012 allows you to use NFS shares to host the database files out of the box. This means no sharing of luns across nodes and allows for faster failover in the event of a disaster. While not "local" disk and still most times on a storage system it is another option for storing database files that does not involve sharing storage across nodes.
Friday, December 14, 2012 - 8:09:09 AM - Perry Whittle
Not a bad article but there are some items to address.
In myth2 you state "the virtual name of SQL Server and the instance of SQL Server are not available until the server recovers ", but this is not entirely true. The virtual networkname has no dependency on sql server only on the IP address, sql server has the dependency on the network name. In reality, the name and IP failover very quickly, or at least as long as it takes to unbind the IP and name from the currently active node, re register and bind it to the new partner node.
Also bear in mind that, when using a clustered DTC resource in flight transactions are a lot safer during a cluster failover.
In myth3 you state "Availability groups are a grouping of databases with a virtual name and IP address that act as a single unit for users to access." but this is not entirely true. When you create an AlwaysOn group it creates a clustered application with the same name in Failover Cluster manager, but its not purely group access related. If you don't create a listener for your AO group, and you really don't have to, the clustered application will be pretty much empty.
For more on creating AO groups with a listener see my article at this link