[ovirt-users] why replica 3

[Simone Tiraboschi]

On Fri, Sep 4, 2015 at 8:56 AM, Richard Neuboeck <hawk at tbi.univie.ac.at>
wrote:

> Hi oVirt group,
>
> I'm currently trying to set up an oVirt 3.6 self hosed engine on a
> replica 2 gluster volume. As you can imagine it fails since it expects a
> replica 3 volume.
>
> Since this part is hard coded during the installation this could be
> easily remedied but on the other hand I'm not familiar with the oVirt
> setup so I'm not sure if I would be disturbing something far a head that
> again is puzzled by the lack of a replica 3 volume.
>
> 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.


> Shouldn't the redundancy of the storage back end be the responsibility
> of the admin and therefore the option to choose (at least replicated and
> distributed replicated) open?
>
> I'm asking because the storage back and I'm using is kind of a 'big'
> thing and it's hardware is put together to facilitate the use of replica
> 2. Sure I could create multiple bricks on those two machines but other
> than a negative performance impact I would gain nothing. Since the price
> tag of one of those servers is quite high the likelihood of getting
> another one is exactly 0. So I'm sure it makes sens to you that I would
> rather choose my own replication level than having one predetermined.
>
> All the best
> Richard
>
>
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