A Disk-Only TS7700 Engine does not attach to a physical backend library; all virtual volumes are stored in the cache. When the allocated free space is less than 1 TB level, the cluster enters the Out of Cache Resources state. Mount operations that have been accepted prior to entering this state will complete and volumes currently mounted can continue to perform any host I/O operation. Any scratch mount operations received while in this state fail with completion and reason codes indicating that the mount could not be processed due to insufficient cache space. Specific volume mount operations received while in this state are allowed, however, any attempt to write to the volume fails with a write-protect error. The cluster exits this state when the allocated free space is more than 3.5 TB. If the cluster is in a grid configuration, the Out of Cache Resources state reflects the composite state of cache in all TS7700 Disk-Only Clusters in the grid.