[DRBD-user] DRBD9: full-mesh and managed resources

Roberto Resoli roberto at resolutions.it
Mon Aug 22 10:43:18 CEST 2016

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Il 18/08/2016 14:03, Veit Wahlich ha scritto:
> Am Donnerstag, den 18.08.2016, 12:33 +0200 schrieb Roberto Resoli:
>> Il 18/08/2016 10:09, Adam Goryachev ha scritto:
>>> I can't comment on the DRBD related portions, but can't you add both
>>> interfaces on each machine to a single bridge, and then configure the IP
>>> address on the bridge. Hence each machine will only have one IP address,
>>> and the other machines will use their dedicated network to connect to
>>> it. I would assume the overhead of the bridge inside the kernel would be
>>> minimal, but possibly not, so it might be a good idea to test it out.
>> Very clever suggestion!
>> Many thanks, will try and report.
> If you try this, take care to enable STP on the bridges, or this will
> create loops.

Yes, this worked immediately as aspected.

> Also STP will give you redundancy in case a link breaks and will try to
> determine the shortest path between nodes.

I confirm. With three nodes and three links of course stp blocks one of
the three links, with root bridge forwording traffing between the other two.

It is possible to control which bridge becomes root using the parameter
"bridgeprio" of brctl.

> But the shortest link is not guaranteed. Especially after recovery from
> a network link failure.
> You might want to monitor each node for the shortest path.

Using stp of course has the side effect of not using one of the three
links (it is the price to pay for failover).

I tried to disable stp, blocking at the same time (with a simple ebtable
rule) the forwardings through the bridge in order to avoid
loops/broadcast storms. In the resulting topology every link carries
only the traffic of the two nodes it connects (at the expense of having
no failover).

it is very handy to monitor that all is working correctly using:

watch brctl showstp <bridge_name>


watch brctl showmacs <bridge_name>

I post here the configuration I ended up to use, for reference:
(I put it in a "drbd-interfaces" file, referenced in
"/etc/network/interfaces" using the "source" directive)

auto drbdbr
iface drbdbr inet static
        address  <ip_of_this_drbd_node>
        bridge_ports eth2 eth3
        bridge_stp off
        bridge_ageing 30
        bridge_fd 5
# Only with stp on
      # node1 and node2 are preferred
        #bridge_bridgeprio 1000
# Only with stp off
	pre-up ifconfig eth2 mtu 9000 && ifconfig eth3 mtu 9000
        up  ebtables -I FORWARD --logical-in drbdbr -j DROP
        down ebtables -D FORWARD --logical-in drbdbr -j DROP


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