[ovirt-users] why replica 3

[Simone Tiraboschi]

On Wed, Sep 9, 2015 at 10:49 AM, Jorick Astrego <j.astrego at netbulae.eu>
wrote:

>
>
> On 09/09/2015 10:14 AM, Richard Neuboeck 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?
> >
> > In my case coherence checks would come from outside the storage and
> > vm host setup and fencing would be applied appropriately.
> >
> > 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
> >
> >
> As a side question, in Glusterfs 3.7 there is an "AFR arbiter volume"
> (
> http://gluster.readthedocs.org/en/release-3.7.0-1/Features/afr-arbiter-volumes/)
>
> that will only contain the metadata.
>
> Will ovirt support this in 3.6?
>

Not explicitly and I never tried it with hosted-engine but I suspect it
works as far it will report  replica level = 3.

By the way you can save disk space on the third host but you still need
three hosts.


> Kind regards,
>
> Jorick Astrego
>
>
>
>
> Met vriendelijke groet, With kind regards,
>
> Jorick Astrego
>
> *Netbulae Virtualization Experts *
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