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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. 

## 10.7.5 Synchronous remote copy 

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. 

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