SQL Server Performance

2xSQL 2005 Instances on one cluster

Discussion in 'SQL Server 2005 Clustering' started by Funkoid, Mar 14, 2008.

  1. Funkoid New Member

    I've been reading through the best clustering practices here
    I've noticed it is not recommended to install two instances of MSSQL in a cluster. What are the reasons for this? I understand a 2nd instance will provide more flexibility with regard to starting & stopping services etc what is the down side of this?

  2. satya Moderator

    SQL Server 2005 can support up to 50 instances when it’s not clustered. When SQL Server is clustered, the maximum number of instances is 25, but this number can be further restricted by additional limitations imposed by clustering
    Sometimes it is possible to just sum the individual resource requirements for CPU, memory, and I/O, and get a reasonably good idea of how many instances can fit.
  3. DevDBA New Member

    We run 3 instances on a 2-node cluster in active-active mode, where one node (under normal conditions) runs our primary OLTP instance, and the other node runs two instances: a "read-only" instance for Reporting and a utility instance that is very small and lightweight.
    Both nodes are capable of running all 3 instances (and then some, to be honest). We've had a couple of hardware failures that left us running on a single node for a week, and it was barely noticeable.
    I think the real issue is with n-Node clusters running many active instances. Could you fail every instance over to just one server? That would mean a huge strain on memory, disk IO, and CPU. Servers capable of that would cost big $$$.
  4. satya Moderator

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