Two IBM Spectrum Virtualize systems must be defined in a partnership, which must be  performed on both systems to establish a fully functional Metro Mirror partnership. By using standard single-mode connections, the supported distance between two systems in  a Metro Mirror partnership is 10 km (6.2 miles), although greater distances can be achieved  by using extenders. For extended distance solutions, contact your IBM representative. Limit:  When a local fabric and a remote fabric are connected for Metro Mirror purposes,  the inter-switch link (ISL) hop count between a local node and a remote node cannot  exceed seven. Metro Mirror is a fully synchronous remote copy technique that ensures that writes are  committed at both the master and auxiliary volumes before write completion is acknowledged  to the host, but only if writes to the auxiliary volumes are possible.  Events, such as a loss of connectivity between systems, can cause mirrored writes from the  master volume and the auxiliary volume to fail. In that case, Metro Mirror suspends writes to  the auxiliary volume and enables I/O to the master volume to continue to avoid affecting the  operation of the master volumes.  Figure 10-70 on page 538 shows how a write to the master volume is mirrored to the cache of  the auxiliary volume before an acknowledgment of the write is sent back to the host that  issued the write. This process ensures that the auxiliary is synchronized in real time if it is  needed in a failover situation. Figure 10-70 Write on volume in Metro Mirror relationship 10.7.5 Synchronous remote copy 538  Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.2.1