Figure 3-18 shows an example of a supported configuration with SAN Volume Controller  nodes using shortwave SFPs. In Figure 3-18, the optical distance between SAN Volume Controller Node 1 and Host 2 is  slightly over 40 km (24.85 miles). To avoid latencies that lead to degraded performance, avoid ISL hops whenever possible. In  an optimal setup, the servers connect to the same SAN switch as the SAN Volume Controller  n ode s. Note:  Before attaching host systems to SAN Volume Controller, see the Configuration  Limits and Restrictions for the IBM System Storage SAN Volume Controller described in: http://www.ibm.com/support/docview.wss?uid=ibm10744461 Typically, hosts issue subsequent I/O requests to storage systems without waiting for  completion of previous ones. The number of outstanding requests is called  queue depth .  Sending multiple I/O requests in parallel (asynchronous I/O) provides significant performance  benefits compared to sending them one-by-one (synchronous I/O). However, if the number of  queued requests exceeds the maximum supported by the storage controller, you will  experience performance degradation. Therefore, for large storage networks you should plan for setting the correct SCSI commands  queue depth on your hosts. For this purpose, a large storage network is defined as one that  contains at least 1000 volume mappings. For example, a deployment with 50 hosts with 20  volumes mapped to each of them would be considered a large storage network. For details of  the queue depth calculations, search for  Queue depth in large SANs  on the following site: https://www.ibm.com/support/knowledgecenter/STPVGU 3.11.1 Queue depth Chapter 3. Planning  79 Figure 3-18 Example of host connectivity