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Can I get a list of disadvantages and advantages for this question, or the correct one to use.

If a company has around 6 websites, each with there on SQL Server Database, should those Database's be on their own Server and SQL Server instance, or can they all exist on one Server under one SQL Server instance under different databases?

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It app depends on the load required by each database. If the IO, memory and CPU load for each one is minimal then they can all sit on a single SQL Server instance. As the requirements increase you'll need to move the instances to a different SQL Server.

The main reason to put the databases on different instances is because the instance have different, conflicting security requirements. Another reason is because you want to control the size of the buffer pool and procedure cache for one database so you would put it on a different instance. Otherwise putting everything in a single instance is just fine.

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I can't see any reason (at least not without more info) as to why they couldn't be under the same SQL instance. We use a consolidated SQL instance model (meaning multiple apps to 1 SQL server) and it works fine.

The disadvantages that I see are:

  1. A little more difficult to troubleshoot performance issues
  2. All your eggs in one basket (unless you cluster / mirror / log ship)
  3. One site sites DB usage "could" affect the others from a CPU / memory perspective.

The advantages that I see are:

  1. Easier to admin (backup for example only needs to be configured once)
  2. Less SQL servers to manage (sort of realted to 1)

As far as I can tell, the 1 app to 1 SQL server model is sort of going by way side. I'm bias and personally a big fan of multiple apps to one SQL server. Just don't skimp on the resources for that SQL server. That's not to say that you need to over spec, understand what performance needs the sites need. Also, as an FYI, all our SQL servers are virtual, incase anyone trys to throw that gotcha at you, we've had no problems. All comes down to proper sizing.

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We run one database instance for "app" data (e.g., Drupal, WP, Mediawiki) and a separate instance for business data. That way there are no potential contention/escalation issues between the two. If you're truly paranoid about interference between applications, I don't think MySQL will give you the level of separation you need resource-wise.

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