fan-in: The ratio of hosts to storage devices. It is the view of the SAN from the storage port's perspective.
See also: fan-out
The concept of fan-in and fan-out occur in many contexts, audio, chips, circuitry, and network, including Storage Area Networks. The value may be hard or soft. A soft value might represent the ratio supported by a vendor, e.g., the support matrix agrees to support HBA connectivity no more than eight (8) device ports, i.e., a fan-in ratio of 1:8. Similarly, it might be a hard limit, usually based on internal address structures. Often, the soft limit is set lower than the hard limit, particularly during product introduction.
The nomenclature is always a bit tricky. In the case of fan-in, the left digit in the ratio should be the number one (1) when stated without context; similarly in the case of fan-out, the right digit should be the number one (1) when stated without context - not all vendors honor this approach, hence the need for context. In hardware engineering circles, many dispense with fan-in and think of everything as fan-out. An easy way to remember fan-in is: looking into the fabric from a host port, how many ports can you access? Thinking of the word "in" is useful because the host that sends requests into the storage. However, there is significant confusion because of the word "perspective" in the standard definitions can lead one into the error of setting a direction that begins from the array port, rather than holding the number of array ports as the unit to be measured.
In the example, we see two servers, a switch and a storage array. All four (4) array ports and one (1) port from each server connect to the switch. Recall that ports in the array and ports in the hosts are all considered to be node ports. The graphic depicts logical paths, the zones, between node ports (in pink). Zones restrict the connectivity pattern between node ports. In the case of the UNIX Server port, only one (1) array node port is visible, therefore the fan-in ratio is 1:1, i.e., one host port : one array port. In the case of the Windows Server port, only two (2) array nodes are visible, therefore the fan-in ratio is 1:2, i.e., one host port : two array ports.
It is useful to recognize that if the HBA could see node ports from other storing devices, or even other hosts, these would be aggregated in the ratio - there is no restriction on the type of device seen. However, this is not a count of the sub-devices, for example disks presented on the channel. Also, fan-in is restrictive. An HBA might offer connection to 2,048 devices with a fan-in ratio of 1:8. Internally, when the ratio is hard, it means that the HBA can manage eight (8) addresses and up to 256 sub-devices within each address.
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