faq: Explain why select groups don't sort out packets evenly.

This keeps coming up.

Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/Documentation/faq/openflow.rst b/Documentation/faq/openflow.rst
index 4a20255da..561d793a6 100644
--- a/Documentation/faq/openflow.rst
+++ b/Documentation/faq/openflow.rst
@@ -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::