Microsoft SQL Server supports multiple instances of the SQL
Server database engine running concurrently on the same computer. Each instance
of the SQL Server database engine has its own set of system and user databases
that are not shared between instances. Applications can connect to each SQL
Server database engine instance on a computer in much the same way they connect
to SQL Server database engine running on different computers.
There are two types of instances of SQL Server:
Default Instances:
The default instance is identified solely by the name of
the computer on which the instance is running; it does not have a separate
instance name. When applications specify only the computer name in their
requests to connect to SQL Server, the SQL Server client components attempt to
connect to the default instance of the database engine on that computer. This
preserves compatibility with existing SQL Server applications.
There can only one default instance on any computer, the
default instance can be any version of SQL Server.
Named Instances:
All instances of the database engine other than te default
instance are identified by an instance name specified during installation of
the instance. Applications must provide both the computer name and the instance
name of any named instance to which they are attempting to connect. The
computer name and instance name are specified in the format
computer_name\instance_name.
There can be multiple named instances running on a
computer, but the earlier versions of SQL Server 2000 cannot operate as a named
instance.
Instances apply primarily to the database engine and its
supporting components, not to the client tools. When you install multiple
instances, each instance gets a unique set of:
- System and user databases.
- The SQL Server and SQL Server Agent services. For default instances, the names of services remain MSSQLServer and SQLServerAgent. For named instances, the names of the services are changed to MSSQL$instancename and SQLAgent$instancename, allowing then to be started and stopped independently of the other manage scheduled events for the associated instances of the database engine.
- The registry keys associated with the database engine and the SQL Server and SQL Server Agent services.
- Network connection addresses so that applications can connect to specific instances.
Shared components:
The following components are shared between all of the
instances running on the same computer since there is only one copy of these
components existed on the computer:
- The SQL Server program group
- Utility represented by each icon in the program group
- SQL Server Books Online
- SQL Server Configuration Manager
- SQL Server Management Studio
- MSSearchService that manages full-text searches against all of the instances of SQL Server on the computer
- There is only one copy of the T-SQL and Microsoft SQL Server Analysis Services servers
- The registry keys associated with the client software are not duplicated between instances
- There is only one copy of the SQL Server development libraries (include and .lib files) and sample applications
| SQL Server Database Engine Version |
Instances per computer on 32-Bit |
Instances per computer on 64-Bit |
|---|---|---|
| V.2012 | 50 instances on a stand-alone server for all SQL Server editions. SQL Server supports 25 instances on a failover cluster when using a shared cluster disk as a stored option for you cluster installation SQL Server supports 50 instances on a failover cluster if you choose SMB file shares as the storage option for your cluster installation |
50 instances on a stand- alone server. 25 instances on a failover cluster when using a shared cluster disk as the stored option for you cluster installation SQL Server supports 50 instances on a failover cluster if you choose SMB file shares as the storage option for your cluster installation |
| V.2008R2 | 50 instances on a stand-alone server for all SQL Server editions. SQL Server supports 25 instances on a failover cluster |
50 instances on a stand- alone server. 25 instances on a failover cluster. |
| V.2008 | 50 instances on a stand-alone server for all SQL Server editions. SQL Server supports 25 instances on a failover cluster |
50 instances on a stand- alone server for all SQL Server editions. SQL Server supports 25 instances on a failover cluster |
| V.2005 | 50 instances on a stand-alone server for all SQL Server 2005 editions except for Workgroup Edition. Workgroup Edition supports a maximum of 16 instances. SQL Server 2005 supports 25 instances on a failover cluster |
50 instances on a stand- alone server. 25 instances on a failover cluster |
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