[Linux-HA] Xen multiple resource instance prevention

[Yann Cezard]

Ivan a écrit :
> Hi List,
>
> I am a bit new to HA but read what was available and now I have a well
> working Xen cluster based on SLES10SP1.
>
> Background:
> My Xen domUs are running off a san, partitions are LVM2 on top of EVMS
> CSM. I cannot have OCFS2 image store for my domUs due to the nature of
> them so must use partitions instead if file images.
> [...]
>
> I use HA 2.0.8 with crm only.
>
> Thanks a lot in advance,
> Ivan
Hi Ivan,

I do not really have the answer to your question (except that : are you
sure 2.0.8 is sufficient to make Xen VM migration ? When I looked at 
this some
months ago, 2.1.X was required... but it could have changed).

The reason I answer to this mail, is that I want to do the same thing as
you, and I am sure that a lot of other people do to (HB2 - Xen - SAN - 
live migration -
resource monitoring - _NO_ ClusterFS).
The only point that block me now is "how to be totally sure that a LUN
won't be mounted with Write access on two nodes at the same time".
Cause I don't want my filesystem to be destroyed in case of a 
split-brain occures.

Using a cluster aware FileSystem is not really a solution, cause I don't
want to make parallel access, I just want that it never happens.

I tried to have a look at CLVM, but it is really hard to find some 
documentation
to it not related to GFS.

It seems that the good way is using some kind of quorum, and so, the minimal
cluster size should be 3 nodes (even if quorum is possible with two 
nodes, by
telling that one node has a biggest weight in quorum).

And than, this is what should happen in case of problem :
- node A,  resources : XenVM1 - XenVM2
- node B,  resources : XenVM3 - XenVM4
- node C,  resources : no Xen VM

Case 1 : node B goes down
- node A,  resources : XenVM1 - XenVM2
- node B,  OUT
- node C,  START resources : XenVM3 - XenVM4

Case 2 : node B looses network and so HB2 cluster connection
- node A,  resources : XenVM1 - XenVM2
- node B,  STOP resources : XenVM3 - XenVM4
- node C,  START resources : XenVM3 - XenVM4 (should happen after
  node B has stopped its resources, but there is no way to know that as B
  is OUT of the HB cluster)

Case 3 : split-brain A, B & C loose network
- node A,  STOP resources : XenVM1 - XenVM2
- node B,  STOP resources : XenVM3 - XenVM4
- node C,  nothing to do

This does not look so hard to achieve but I wonder if I am missing some 
other
problems that could happened, or perhaps am I going in the wrong direction ?

I think this could be interesting to have some web resources about
HB2 + Xen on the linux-ha.org website, more and more people will try
to do such cluster configuration.

So if anyone has some interest in this, some clue, or just want to give 
his/her opinion,
he/she's welcome.

Thanks,

Yann