[ovirt-users] Hyperconverged Setup and Gluster healing

[FERNANDO FREDIANI]

Ok, great, thanks for the clarification.

Therefore a replica 3 configuration means raw storage space cost is 
'similar' to a RAID 1 and actual data exists only 2 times and two 
different servers.

Regards
Fernando


On 24/04/2017 11:35, Denis Chaplygin wrote:
> With arbiter volume you still have a replica 3 volume, meaning that 
> you have three participants in your quorum. But only two of those 
> participants keep the actual data. Third one, the arbiter, stores only 
> some metadata, not the files content, so data is not replicated 3 times.
>
> On Mon, Apr 24, 2017 at 3:33 PM, FERNANDO FREDIANI 
> <fernando.frediani at upx.com <mailto:fernando.frediani at upx.com>> wrote:
>
>     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/
>>     <https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/>
>
>

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