faq: Explain why select groups don't sort out packets evenly.
This keeps coming up.
Acked-by: Alin Gabriel Serdean
@@ -478,6 +478,22 @@ Q: How does OVS divide flows among buckets in an OpenFlow "select" group?
Documentation/group-selection-method-property.txt in the Open vSwitch
source tree. (OpenFlow 1.5 support in Open vSwitch is still experimental.)
+Q: An OpenFlow "select" group isn't dividing packets evenly among the buckets.
+
+ A: When a packet passes through a "select" group, Open vSwitch hashes a
+ subset of the fields in the packet, then it maps the hash value to a
+ bucket. This means that packets whose hashed fields are the same will
+ always go to the same bucket[*]. More specifically, if you test with a
+ single traffic flow, only one bucket will receive any traffic[**].
+ Furthermore, statistics and probability mean that testing with a small
+ number of flows may still yield an uneven distribution.
+
+ [*] Unless its bucket has a watch port or group whose liveness changes
+ during the test.
+
+ [**] Unless the hash includes fields that vary within a traffic flow, such
+ as tcp_flags.
+
Q: I added a flow to accept packets on VLAN 123 and output them on VLAN 456,
like so::
