content_image/997803.jpg
Figure 3-18 shows an example of a supported configuration with SAN Volume Controller nodes using shortwave SFPs. 

<img src='content_image/997803.jpg'>

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: 

## 3.11.1 Queue depth 

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 

