Tip:  SCSI write over FC requires two round trips per I/O operation, as shown in the  following example: Each SCSI I/O has 1 ms of more service time. At 100 km, it becomes 2 ms for more  service time. At 50 km, you have another latency, as shown in the following example: 2 (round trips) x 2 (operations) x 5 microsec/km = 20 microsec/km 20 microsec/km x 50 km = 1000 microsec = 1 msec (msec represents millisecond) Link bandwidth  is the network capacity to move data as measured in millions of bits per  second (Mbps) or billions of bits per second (Gbps). The term  bandwidth  is also used in the following context: Intersystem connectivity  supports mirrored foreground and background I/O. A portion of the  link is also used to carry traffic that is associated with the exchange of low-level messaging  between the nodes of the local and remote systems. A  dedicated amount  of the link  bandwidth is required for the exchange of heartbeat messages and the initial configuration of  intersystem partnerships. Interlink bandwidth must support the following traffic: Fibre Channel  connectivity is the standard connectivity that is used for the remote copy  intersystem networks. It uses the Fibre Channel protocol and SAN infrastructures to  interconnect the systems. Native IP  connectivity has been introduced with IBM Spectrum Virtualize version 7.2 to  implement intersystem networks by using standard TPC/IP infrastructures. The maximum supported round-trip latency between sites depends on the type of partnership  between systems. Table 6-6 on page 180 lists the maximum round-trip latency. This  restriction applies to all variants of remote mirroring.  More configuration requirements and guidelines apply to systems that perform remote  mirroring over extended distances, where the round-trip time is greater than 80 ms. If you use  remote mirroring between systems with 80 - 250 ms round-trip latency, you must meet the  following additional requirements: GLYPH<SM590000> Storage bandwidth: The ability of the back-end storage to process I/O. Measures the  amount of data (in bytes) that can be sent in a specified amount of time. GLYPH<SM590000> Remote copy partnership bandwidth (parameter): The rate at which background write  synchronization is attempted (unit of MBps).  GLYPH<SM590000> Mirrored foreground writes, as generated by foreground processes at peak times GLYPH<SM590000> Background write synchronization, as defined by the Global Mirror bandwidth parameter GLYPH<SM590000> Intersystem communication ( heartbeat messaging ) Network latency considerations Table 6-6 Maximum round trip Partnership FC 1 Gbps IP 10 Gbps IP 250 ms 80 ms 10 ms 180  IBM FlashSystem 9200 and 9100 Best Practices and Performance Guidelines