[DRBD-user] Replication problems constants with DRBD 8.3.10

Digimer lists at alteeve.ca
Sun Jun 16 20:07:51 CEST 2013

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

On 06/16/2013 11:39 AM, Adam Goryachev wrote:
> On 16/06/13 18:47, Luca Fornasari wrote:
>> On Sun, Jun 16, 2013 at 6:44 AM, cesar <brain at click.com.py
>> <mailto:brain at click.com.py>> wrote:
>>     And about DRBD over a direct connection in mode round robin, can
>>     you give me
>>     links or comments about this case? (This is very important for me
>>     because I
>>     will lose connection speed if I change of balance-rr to
>>     active-backup).
>> https://www.kernel.org/doc/Documentation/networking/bonding.txt
>> balance-rr: This mode is the only mode that will permit a single
>> 	TCP/IP connection to stripe traffic across multiple
>> 	interfaces. It is therefore the only mode that will allow a
>> 	single TCP/IP stream to utilize more than one interface's
>> 	worth of throughput.  This comes at a cost, however: the
>> 	striping generally results in peer systems receiving packets out
>> 	of order, causing TCP/IP's congestion control system to kick
>> 	in, often by retransmitting segments.
>> The problem is you have out of order packets and it doesn't help if
>> you start to play around with net.ipv4.tcp_reordering sysctl parameter
>> because there will always be a chance to have out of order packets.
>> Ordered packets are indeed fundamental to DRBD.
>> In DRBD world the bonding driver is used to achieve HA using
>> active/backup or 802.3ad. Neither of which will boost your performance
>> (802.3ad can improve performance if and only if you have a great
>> number of TCP connections but that's not the case with your DRBD
>> scenario).
> Hi,
> It was my understanding that this was only the case if switches were
> involved, and especially different switches. When you have 2 (or more
> ports) on one machine connected via crossover cables to another server
> with an equal number of ports (obviously), and you configure both sides
> with RR, then all packets should arrive in order.
> Since packet 1 is sent on eth0, and packet 2 on eth1 from server 1
> Server 2 will receive packet 1 on eth0 and packet 2 on eth1 in the same
> order.
> If you had switch1 between the two machines on eth0, and switch2 between
> the machines on eth1, then it is very possible the switch will introduce
> different amounts of delay, therefore causing out of order packets.
> At least, this was my understanding after some recent detailed
> reading/research into these matters. If this is still wrong, even for a
> set of direct cross over connections, please educate both me and the OP.
> Regards,
> Adam

Having identical hardware in back-to-back config maximizes the chance 
that the packets will arrive in order, but it does not guarantee it.

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