It is possible to have more than one instance on a Linux or UNIX operating system if the DB2® product was installed with root privileges. Although each instance runs simultaneously, each is independent. Therefore, you can only work within one instance of the database manager at a time.
The instance owner and the group that is the System Administration (SYSADM) group are associated with every instance. The instance owner and the SYSADM group are assigned during the process of creating the instance. One user ID or username can be used for only one instance, and that user ID or username is also referred to as the instance owner.
Each instance owner must have a unique home directory. All of the configuration files necessary to run the instance are created in the home directory of the instance owner's user ID or username. If it becomes necessary to remove the instance owner's user ID or username from the system, you could potentially lose files associated with the instance and lose access to data stored in this instance. For this reason, you should dedicate an instance owner user ID or username to be used exclusively to run the database manager.
The primary group of the instance owner is also important. This primary group automatically becomes the system administration group for the instance and gains SYSADM authority over the instance. Other user IDs or usernames that are members of the primary group of the instance owner also gain this level of authority. For this reason, you might want to assign the instance owner's user ID or username to a primary group that is reserved for the administration of instances. (Also, ensure that you assign a primary group to the instance owner user ID or username; otherwise, the system-default primary group is used.)
If you already have a group that you want to make the system administration group for the instance, you can assign this group as the primary group when you create the instance owner user ID or username. To give other users administration authority on the instance, add them to the group that is assigned as the system administration group.
To separate SYSADM authority between instances, ensure that each instance owner user ID or username uses a different primary group. However, if you choose to have a common SYSADM authority over multiple instances, you can use the same primary group for multiple instances.