A remirror buffer can be used in failover situations so as to backup storage volumes in a service provider. The remirror buffer is dynamically resized to meet current usage metrics captured from a data center. A risk boundary can be defined through which resource hosts are grouped together so as to determine the usage metrics. The risk boundary can be based on a topology of the data center, such as a room/rack/sharing of power supplies, or other characteristics of the resource hosts.