Most physical file systems (PFSes) allow only one connection for update at a time. Such file systems are called non-sysplex aware for update. This is directly related to the mount mode of the file system. With HFS, for example, only one system can actually connect to the file system with a mode of RDWR. That system is called the file system owner.

The other systems that want to participate in shared file systems will also request an RDWR mount, but their access will be provided via cross-system messaging with the file system owner which has already established the read-write connection. These systems are called file system clients. When the file system owner becomes unavailable (for example, through system shutdown), it will be important for another system (one of the file system clients) to have the file system volume varied online so that a new owner can be established. This helps ensure maximum file system availability in the shared file system group.