[ovirt-users] Hyperconverged Setup and Gluster healing

[FERNANDO FREDIANI]

But then quorum doesn't replicate data 3 times, does it ?

Fernando


On 24/04/2017 10:24, Denis Chaplygin wrote:
> Hello!
>
> On Mon, Apr 24, 2017 at 3:02 PM, FERNANDO FREDIANI 
> <fernando.frediani at upx.com <mailto:fernando.frediani at upx.com>> wrote:
>
>     Out of curiosity, why do you and people in general use more
>     replica 3 than replica 2 ?
>
>
> The answer is simple - quorum. With just two participants you don't 
> know what to do, when your peer is unreachable. When you have three 
> participants, you are able to establish a majority. In that case, when 
> two partiticipants are able to communicate, they now, that lesser part 
> of cluster knows, that it should not accept any changes.
>
>     If I understand correctly this seems overkill and waste of storage
>     as 2 copies of data (replica 2) seems pretty reasonable similar to
>     RAID 1 and still in the worst case the data can be replicated
>     after a fail. I see that replica 3 helps more on performance at
>     the cost of space.
>
>
> You are absolutely right. You need two copies of data to provide data 
> redundancy and you need three (or more) members in cluster to provide 
> distinguishable majority. Therefore we have arbiter volumes, thus 
> solving that issue [1].
>
> [1] 
> https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/

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