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SQL - What is an SQL Instance? What is an SQL Cluster? Are these related?

What is an SQL instance?
What is an SQL cluster?
The web descriptions are not really clear and don't  give any examples!!!
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brothertruffle880
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brothertruffle880
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4 Solutions
 
Matt VCommented:
A SQL instance is a single install of SQL.  It is installed in it's own folder.
An example would be installing SQL 2000 and then installing SQL 2008 into a new folder.  Both are still running, and each is an instance.  You can also install multiple instances of the same version by specifying a new folder for each install.

A SQL cluster is a Microsoft cluster that also runs SQL server.  The SQL instances are clustered into an active-passive (active fails, other node takes over) or active-active (both nodes are active at the same time).
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65tdCommented:
A single SQL instance in cluster would have it's own IP address, network name and disk(s) and could only be run as an active/passive cluster.

Multiple instances of SQL which own (again) have it's own network name , ip address and disk(s), could be run as an active/active SQL cluster.
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brothertruffle880Author Commented:
What is the rationale for having multiple instances rather than just creating copies of a database? or creating another SQL Server?
I'm trying to determine when to create an instance versus when to just copy the database or install SQL server all over again in a different box?

The answers I found on the web are a bit hazy on the rationale for setting up instances of SQL.
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Matt VCommented:
If you have a very powerful server, you can do processor based licensing and install multiple instances on one server instead of licensing multiple physical servers.

You can also install mutiple versions as different instances if you have version requirements for your applications.

Usually it is for utilizing hardware better or consolodating licenses.
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brothertruffle880Author Commented:
Hi 65TD:

What is an Active/Passive cluster?
When would you use it?

BT
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Matt VCommented:
Active/Passive means that all the databases reside on one server (active) and only fail over to the other server (passive) if the active one goes down.

This is done to save licencing.  You only need the SQL enterprise licence for the active node in this scenario.

This provides some fault tolerance (you are back up quicker than complete hardware failure of one server), without the additional licencing costs.

You can also migrate the databases from the active to the passive node for patching.  As long as you only have databases on one node you are still properly licenced.
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