Re: Multiple load balancers problem

To: Julian Anastasov <ja@xxxxxx>
Subject: Re: Multiple load balancers problem
Cc: lvs-devel@xxxxxxxxxxxxxxx
From: Dmitry Akindinov <dimak@xxxxxxxxxxx>
Date: Tue, 28 Aug 2012 00:24:38 +0400

On 2012-08-27 20:13, Julian Anastasov wrote:


On Mon, 27 Aug 2012, Dmitry Akindinov wrote:


Sorry for top posting: doing this to avoid clutter below.

Thank you for your assistance. Let me to summarize the current situation,
after all the changes and testing.

1. The test system consists of two servers, S1 and S2. Both running CentOS

  [root@fm1 ~]# uname -a
  Linux fm1.***.com 2.6.32-71.el6.x86_64 #1 SMP Fri May 20 03:51:51 BST 2011
  x86_64 x86_64 x86_64 GNU/Linux

We are now setting up new boxes (CentOS 6.3) to re-test with newer kernels

        The last bug fixes that can be needed for sync
are from April 2012.

2. Both systems have iptables configured to mark the traffic to VIP with the
"100" marker.

3. At the beginning of the test,
IPSV on S1 configuration is:
-A -f 100 -s rr -p 1
-a -f 100 -r S1:0 -g -w 100
-a -f 100 -r S2:0 -g -w 100

IPSV on S2 configuration is:
-A -f 100 -s rr -p 1
-a -f 100 -r S1:0 -g -w 0
-a -f 100 -r S2:0 -g -w 100

We establish test connections from client systems to the port 110 of VIP, the
S1 routes one connection to itself, the other one to S2. Both connections are
alive and well, the connection tables on both systems are the same due to ipsv
syncing daemons:

ipsvadm -l -c -n:
IP  00:49  NONE        C1:0        S1:0
TCP 14:49  ESTABLISHED C1:54837 VIP:110   S1:110
TCP 14:43  ESTABLISHED C2:54648 VIP:110   S2:110
IP  00:43  NONE       C2:0         S2:0

Now, we initiate a failover, so S2 becomes the active balancer.
The IPSV rules on S2 are updated, so they become the same as they were on S1:
-A -f 100 -s rr -p 1
-a -f 100 -r S1:0 -g -w 100
-a -f 100 -r S2:0 -g -w 100

And S1 gets the same config as S2 used before the failover.

All the connections that existed on S2 before failover continue to work.

        No loop? Because S2 will receive DR method in
the SYNC messages for its stack. You see traffic flow
or just that there is no reset?

Yes, after the failover, all connections that were open on the S2 (which now becomes the active balancer) do not reset and continue to function just fine (traffic flow in both directions).

We do understand (we think) your explanation about the sync table problem - the connections to the actual balancer are marked in a special way, which causes problems when these connection table records are used on a new balancer.

It looks like updating the kernels is the only way, if (as you and Hans Schillstrom outlines) even the latest CentOS/RedHat kernels do not contain necessary patches. It's a pity, as the idea was to provide an "out of the box" solution for our customers, and asking them to update to some Linux kernel is not what they like to hear.

But thank you very much in any case, we will update the kernels on our test systems and will see if this (last?) problem disappears.

But the connections that existed on S1 are closed as soon as the client sends

        This is fixed by the mentioned change:
"ipvs: changes for local real server"

        Before this change the SYNC messages indicate
LOCALNODE as forwarding method. So, S2 thinks that the
conns for S1 stack are LOCALNODE, S2 deliver them locally
and they are reset.

any data to that connection. The tcpdump on S1 does not show any incoming
packets, and tcpdump on S2 shows that it's S2 itself (a new load balancer)
that closes these connections (the data the client has sent was "HELP\r\n" - 6

07:54:59.214200 IP (tos 0x10, ttl 54, id 20406, offset 0, flags [DF], proto
TCP (6), length 58)
     C1.54837 > VIP.110: Flags [P.], cksum 0xba0d (correct), seq
3572724860:3572724866, ack 1018696840, win 33304, options [nop,nop,TS val
3371318384 ecr 1243703099], length 6
07:54:59.214253 IP (tos 0x10, ttl 64, id 0, offset 0, flags [DF], proto TCP
(6), length 40)
     VIP.110 > C1.54837: Flags [R], cksum 0x5767 (correct), seq 1018696840, win
0, length 0

What can cause the new load balancer to reset (Flags [R.]) the existing
connections to the "old" balancer?

        The wrong forwarding type. That is why we now
save the original forwarding type (DR), we sync it and
finally when packet is transmitted if the destination is
local address we deliver it to local stack. We do not
send LOCALNODE type in messages to backups anymore, we
removed this forwarding type.

New connections now work fine, being distributed by the new load balancer to
itself and to the old balancer.


Julian Anastasov <ja@xxxxxx>

Best regards,
Dmitry Akindinov

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