3.6.0 MxDB for SQL Server Installation and Administration Guide (5697-7088, December 2007)

Chapter 2: Introduction 6
Copyright © 1999-2007 PolyServe, Inc. All rights reserved.
The following configuration also has one node providing a backup for the
Virtual SQL Servers on other nodes. In this example, the SQL instances
associated with the Virtual SQL Servers all have the same name,
“instance1.”
This configuration supports only one failover at a time. If the Virtual SQL
Server on node 1 (“virtual1”) fails over and node 2 then goes down, the
Virtual SQL Server on node 2 (“virtual2”) will not be able to fail over. This
occurs because “instance1” on node 4 is currently providing access to the
databases for the “instance1” associated with the Virtual SQL Server
“virtual1.”
A similar situation occurs when a Virtual SQL Server is moved to
maintenance mode (for example, to apply a service pack). In maintenance
mode, all SQL instances associated with the Virtual SQL Server are shut
down on the primary and backup nodes. In the above example, if the
Virtual SQL Server on node 1 (“virtual1”) is moved to maintenance mode,
“instance1” on node 1 and also its backup “instance1” on node 4 will be
shut down. There is now no backup available for “instance1” on node 2
and node 3. To minimize the time without a backup, we recommend that
you perform the maintenance on the backup instance first and then
Node 2
Primary: virtual2
instance2
Node 1
Primary: virtual1
instance1
Node 3
Primary: virtual3
instance3
Node 4
Backup: virtual1,
virtual2, virtual3
instance1
instance2
instance3
Node 2
Primary: virtual2
instance1
Node 1
Primary: virtual1
instance1
Node 3
Primary: virtual3
instance1
Node 4
Backup: virtual1,
virtual2, virtual3
instance1