[ovirt-users] why replica 3

[Simone Tiraboschi]

On Wed, Sep 9, 2015 at 10:14 AM, Richard Neuboeck <hawk at tbi.univie.ac.at>
wrote:

> On 04.09.15 10:02, Simone Tiraboschi wrote:
> >     Is there a reason why it has to be exactly replica 3?
> >
> >
> > To have a valid quorum having the system being able to decide witch is
> > the right and safe copy avoiding an issue called split brain.
> > Under certain circumstances/issues (network issue, hosts down or
> > whatever could happen) the data on different replica could diverge: if
> > you have two and just two different hosts that claim each other
> that its
> > copy is the right one there is no way to automatically take the right
> > decision. Having three hosts and setting the quorum according to that
> > solves/mitigates the issue.
>
>
> Thanks for the explanation. I do understand the problem but since
> I'm somewhat limited in my hardware options is there a way to
> override this requirement? Meaning if I change the checks for
> replica 3 in the installation scripts does something else fail on
> the way?
>

I'm advising that it's not a safe configuration so it's not recommended for
a production environment.
Having said that, as far as I know it's enforced only in the setup script
so tweaking it should be enough.
Otherwise, if you have enough disk space, you can also have a different
trick: you could create a replica 3 volume with 2 bricks from a single host.
It's not a safe procedure at all cause you still have only 2 hosts, so it's
basically just replica 2, and in case of split brain the host with two
copies will win by configuration which is not always the right decision.


> In my case coherence checks would come from outside the storage and
> vm host setup and fencing would be applied appropriately.
>

Can I ask how?


> I would very much appreciate it if the particulars of the storage
> setup could be either selected from a list of possibilities or be
> ignored and just a warning be issued that this setup is not recommended.
>
> Thanks!
> Richard
>
>
> --
> /dev/null
>
>
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