On Mon, Apr 24, 2017 at 3:02 PM, FERNANDO FREDIANI <
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].

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