[ovirt-users] Re: Need advice using pacemaker on VMs for application HA
On Thu, May 24, 2018 at 2:20 PM wodel youchi <wodel.youchi(a)gmail.com> wrote:
Hi,
Excuse the English :)
Need some advice regarding the use of high availability software on VMs on
oVirt.
We are migrating a physical high available application to oVirt.
The HA platform uses pacemaker, it contains two nodes and a shared
storage, fence (stonith) is configured to use ILO.
I know that oVirt offers HA for VM, but this HA is not application aware,
if a service crashes on the VM, it will not be detected.
Fencing could be achieved by fence agent rhev.
My questions are about the best way to migrate this platform to oVirt.
- Is it a good idea to make both VMs (formally nodes) Highly-Available
VMs? or may be pin each one of them to a particular hypervisor and/or use
VM-Affinity?
If the VM should be highly available, you should not pin them to any host.
Pinning them will make sure the vm will *not* be available when the host
is down :-)
So you probably want to use HA VM - with a VM lease.
Warning: do not use HA VM without a VM lease, this will make sure that
you will have split-brain eventually.
- I am thinking about the situation where the hypervisor containing
one of
the VMs crashes, what will be the behavior of the the fence agent on the
application?
Not sure what do you mean by "crashes".
If the hypervisor lost power, HA VM with a VM lease will be started on
another
hypevisor.
The guest agent on the VM will not be able to do anything since it is not
running :-)
- if the crashed VM is not HA, it will not start on another
hypervisor,
so the fence agent will try to fence a VM that does not exist anymore, and
it will stuck.
- if the crashed VM is HA, it will be started on another hypervisor,
but what will happen with the fence agent? I think that one VM will fence
the other one, and the application will still be unreachable for a longer
period.
Not clear what fence agent are you talking about.
- What about the shared storage, we will use a shared disk on oVirt
which
does not support snapshot
What is the question?
- What are the things to avoid?
I think in general, don't try to have two mechanisms that try to do the
same.
Either use your HA solution or oVirt HA solution, but not both in the same
time.
Nir
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