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


On 24/04/2017 10:24, Denis Chaplygin wrote:

On Mon, Apr 24, 2017 at 3:02 PM, FERNANDO FREDIANI <fernando.fredi...@upx.com <mailto:fernando.fredi...@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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