If all servers within a deployment have the same access to file systems exposed by the filer, Oracle VM Manager understands these exports to be uniform. In some cases, an NFS file server may be configured to export mount points with various access restrictions. In the case where some Oracle VM Servers in a deployment are not able to access exports that are available to others, Oracle VM Manager understands these exports to be non-uniform.

Since Oracle VM Manager never connects directly to any storage, but relies on Oracle VM Servers to handle the administration and the discovery of file systems exported by the NFS file server, Oracle VM Manager requires that you designate an Oracle VM Server as an administrative server. In the case of uniform exports, Oracle VM Servers designated as administration servers may be used for the purpose of performing file system refreshes. However, in the case where exports are non-uniform, an Oracle VM Server must be designated for each unique view of the file server. These servers are called 'refresh servers' and should only be configured in the instance where an NFS file server is configured to have non-uniform exports. Attempting to configure refresh servers for an NFS file server with uniform exports is prevented and an error message is returned. Equally, it is not possible to change the export type from non-uniform to uniform if there are any refresh servers defined for the file server. Instead, you must remove all refresh servers from the configuration for the file server, before attempting to change the export type. This design helps to ensure that a configuration behaves optimally for uniform exports, while ensuring that all file systems are refreshes for non-uniform exports.

During a file system refresh for an NFS file server with uniform exports, an Oracle VM Server designated as an administrative server is selected to perform the file system refresh; however in the case of non-uniform exports, a file system refresh is performed by every single Oracle VM Server that is designated as a refresh server. Oracle VM Manager then uses the information reported by each server to construct its view of all of the file systems that are available. For this reason, file system refreshes on NFS file servers configured with non-uniform exports may take longer to complete.

Non-uniform exports are not intended for servers within the same server pool. All of the servers that belong to a server pool that requires access to file systems on an NFS file server with non-uniform exports must be added to an access group. See Access Groups Perspective in the Oracle VM Manager User's Guide for more information on this.