On Thu, Nov 01, 2012 at 10:06:55AM -0700, Justin Pettit wrote:
> Signed-off-by: Justin Pettit <[email protected]>
This is really good (thank you!), but I think it can be improved.
First, I don't think it's a good idea to emphasize a kernel/user split
here. That's an implementation detail that I don't really want to
expose to our users more than we have to, especially since we might
want to change the split in some way later. (Maybe a netmap-like
interface will turn out to be really fast and awesome, etc.) And the
term "userspace" is probably better written "OpenFlow", really.
Here is my suggestion:
Q: Why are there so many different ways to dump flows?
A: Open vSwitch uses different kinds of flows for different purposes:
- OpenFlow flows are the most important kind of flow. OpenFlow
controllers use these flows to define a switch's policy.
OpenFlow flows support wildcards, priorities, and multiple
tables.
When in-band control is in use, Open vSwitch sets up a few
"hidden" flows, with priority higher than a controller or the
user can configure, that are not visible via OpenFlow. (See
the "Controller" section of the FAQ for more information
about hidden flows.)
- The Open vSwitch software switch implementation uses a second
kind of flow internally. These flows, called "exact-match"
or "datapath" or "kernel" flows, do not support wildcards or
priorities and comprise only a single table, which makes them
suitable for caching. OpenFlow flows and exact-match flows
also support different actions and number ports differently.
Exact-match flows are an implementation detail that is
subject to change in future versions of Open vSwitch. Even
with the current version of Open vSwitch, hardware switch
implementations do not necessarily use exact-match flows.
Each of the commands for dumping flows has a different purpose:
- "ovs-ofctl dump-flows <br>" dumps OpenFlow flows, excluding
hidden flows. This is the most commonly useful form of flow
dump. (Unlike the other commands, this should work with any
OpenFlow switch, not just Open vSwitch.)
- "ovs-appctl bridge/dump-flows <br>" dumps OpenFlow flows,
including hidden flows. This is occasionally useful for
troubleshooting suspected issues with in-band control.
- "ovs-dpctl dump-flows [dp]" dumps the exact-match flow table
entries for a Linux kernel-based datapath. In Open vSwitch
1.10 and later, ovs-vswitchd later merges multiple switches
into a single datapath, so it will show all the flows on all
your kernel-based switches. This command can occasionally be
useful for debugging.
- "ovs-appctl dpif/dump-flows <br>", new in Open vSwitch 1.10,
dumps exact-match flows for only the specified bridge,
regardless of the type.
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