Identity Manager stores all of its provisioning and state information in the Identity Manager repository.
The availability of the database instance storing the Identity Manager repository is the most critical piece to achieving a highly available Identity Manager deployment. The repository is the representation of the entire Identity Manager installation and the data within it must be protected as with other important database applications. At minimum, regular backups must be performed.
Do not host the Identity Manager repository on a virtual platform such as a VMware virtual machine because performance (transactions per second) will be hindered significantly.
There can only be one image of the repository. It is not possible to have two separate databases for Identity Manager and attempt to synchronize them nightly. Sun recommends using your database's clustering or mirroring capabilities to provide fault tolerance.