On 26/10/14 12:32 PM, Andrei Borzenkov wrote:
В Sun, 26 Oct 2014 12:01:03 +0100
Vladimir <m...@foomx.de> пишет:
On Sat, 25 Oct 2014 19:11:02 -0400
Digimer <li...@alteeve.ca> wrote:
On 25/10/14 06:35 PM, Vladimir wrote:
On Sat, 25 Oct 2014 17:30:07 -0400
Digimer <li...@alteeve.ca> wrote:
On 25/10/14 05:09 PM, Vladimir wrote:
Hi,
currently I'm testing a 2 node setup using ubuntu trusty.
# The scenario:
All communication links betwenn the 2 nodes are cut off. This
results in a split brain situation and both nodes take their
resources online.
When the communication links get back, I see following behaviour:
On drbd level the split brain is detected and the device is
disconnected on both nodes because of "after-sb-2pri disconnect"
and then it goes to StandAlone ConnectionState.
I'm wondering why pacemaker does not let the resources fail.
It is still possible to migrate resources between the nodes
although they're in StandAlone ConnectionState. After a split
brain that's not what I want.
Is this the expected behaviour? Is it possible to let the
resources fail after the network recovery to avoid fürther data
corruption.
(At the moment I can't use resource or node level fencing in my
setup.)
Here the main part of my config:
#> dpkg -l | awk '$2 ~ /^(pacem|coro|drbd|libqb)/{print $2,$3}'
corosync 2.3.3-1ubuntu1
drbd8-utils 2:8.4.4-1ubuntu1
libqb-dev 0.16.0.real-1ubuntu3
libqb0 0.16.0.real-1ubuntu3
pacemaker 1.1.10+git20130802-1ubuntu2.1
pacemaker-cli-utils 1.1.10+git20130802-1ubuntu2.1
# pacemaker
primitive drbd-mysql ocf:linbit:drbd \
params drbd_resource="mysql" \
op monitor interval="29s" role="Master" \
op monitor interval="30s" role="Slave"
ms ms-drbd-mysql drbd-mysql \
meta master-max="1" master-node-max="1" clone-max="2"
clone-node-max="1" notify="true"
Split-brains are prevented by using reliable fencing (aka stonith).
You configure stonith in pacemaker (using IPMI/iRMC/iLO/etc,
switched PDUs, etc). Then you configure DRBD to use the
crm-fence-peer.sh fence-handler and you set the fencing policy to
'resource-and-stonith;'.
This way, if all links fail, both nodes block and call a fence. The
faster one fences (powers off) the slower, and then it begins
recovery, assured that the peer is not doing the same.
Without stonith/fencing, then there is no defined behaviour. You
will get split-brains and that is that. Consider; Both nodes lose
contact with it's peer. Without fencing, both must assume the peer
is dead and thus take over resources.
That split brains can occur in such a setup that's clear. But I
would expect pacemaker to stop the drbd resource when the link
between the cluster nodes is reestablished instead of continue
running it.
DRBD will refuse to reconnect until it is told which node's data to
delete. This is data loss and can not be safely automated.
Sorry if maybe described it unclear but I don't want pacemaker to do an
automatic split brain recovery. That would not make any sense to me
either. This decission has to be taken by an administrator.
But is it possible to configure pacemaker to do the following?
- if there are 2 Nodes which can see and communicate with each other
AND
- if their disk state is not UpToDate/UpToDate (typically after a split
brain)
- then let drbd resource fail because something is obviously broken and
an administrator has to decide how to continue.
This would require resource agent support. But it looks like current
resource agent relies on fencing to resolve split brain situation. As
long as resource agent itself does not indicate resource failure,
there is nothing pacemaker can do.
This is why stonith is required in clusters. Even with quorum, you
can't assume anything about the state of the peer until it is
fenced, so it would only give you a false sense of security.
Maybe I'll can use resource level fencing.
You need node-level fencing.
I know node level fencing is more secure. But shouldn't resource level
fencing also work here? e.g.
(http://www.drbd.org/users-guide/s-pacemaker-fencing.html)
Currently I can't use ipmi, apc switches or a shared storage device
for fencing at most fencing via ssh. But what I've read this is also not
recommended for production setups.
You could try meatware stonith agent. This does exactly what you want -
it freezes further processing unless administrator manually declares one
node as down.
Support for this was dropped in the Red Hat world back in 2010ish
because it was so easy for a panicy admin to clear the fence without
adequately ensuring the peer node had actually been turned off. I
*strongly* recommend against using manual fencing. If your nodes have
IPMI, use that. It's super well tested.
--
Digimer
Papers and Projects: https://alteeve.ca/w/
What if the cure for cancer is trapped in the mind of a person without
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