When high availability is enabled, database passivation is used as the default passivation method, in which the database handles all storage requirements.
However, if you decide to instead use filesystem passivation, you must select a shared storage solution and need to implement the shared storage API to integrate with the Pega Platform. This is because application servers require fault-tolerant shared storage in order to facilitate initiated and uninitiated shutdowns when using filesystem passivation. The Pega Platform supports shared storage using Network File Storage (NFS) or shared disk out of the box when using filesystem passivation, but these solutions are not inherently fault tolerant.
If custom shared storage implementations are required, the following plug-in for shared storage must be used: com.pega.pegarules.pub.session.CustomPassivationMechanism
For details about using this plug-in, see PDN article Creating a custom passivation method.