[DRBD-user] bond for drbd identical performance with one link down

Ben Timby btimby at gmail.com
Thu May 20 21:26:05 CEST 2010

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


My point was that balance-rr is the ONLY mode to provide multiple link
aggregation for a single tcp stream. That said, the switch's
cooperation is only necessary when a switch is present. The
documentation spells out why.

As both interfaces share the same MAC address, the switch will
recognize only one of them, thus only forwarding packets to it.
Trunking/etherchannel will allow the switch to direct traffic to both
ports. It would seem this problem would not exist if a switch is not
present, however, I have never tested that scenario personally.

The documentation also mentions that with this mode, packets can
arrive out-of-order, requiring reassembly which will eat into any
throughput gain otherwise realized. Again, whether this problem exists
without a switch is unknown to me.

You can use tcpdump to sniff the underlying interfaces. Pay attention
to the sequence numbers and timestamps, IIRC, if you view the dump in
wireshark, it will helpfully show you out-of-order segments.

The documentation I linked discusses this condition along with some
possible mitigating strategies. I tested this configuration a few
years back, and was able to get around 1.2Gbps for a single stream out
of two bonded 1Gbps links. Of course I was using a switch for my
testing, as I had more that two machines involved.



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