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

From: Bart Coninckx <bart.coninckx_at_nospam>
Date: Thu May 20 2010 - 19:52:21 GMT
To: Ben Timby <btimby@gmail.com>

On Thursday 20 May 2010 21:26:05 Ben Timby wrote:
> 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.
>

Since testing with a switch resulted in even poorer results, I'll shy away
from that for now. Will have a go at investigating with Wireshark though
seeing the problem will probably not indicate a solution right of the bat.

Cheers,

B.
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