From fernando.frediani at upx.com Mon Apr 24 13:33:06 2017 Content-Type: multipart/mixed; boundary="===============1316055829292264022==" MIME-Version: 1.0 From: FERNANDO FREDIANI To: users at ovirt.org Subject: Re: [ovirt-users] Hyperconverged Setup and Gluster healing Date: Mon, 24 Apr 2017 10:33:01 -0300 Message-ID: <75dade04-4d98-1329-00eb-8d4a9ea261b3@upx.com> In-Reply-To: CANVzE5kfHSF15iwRYxagU-rSEvN60XSZcjc8jVSTrKdYuXeeEg@mail.gmail.com --===============1316055829292264022== Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable This is a multi-part message in MIME format. --------------B6FFA6AC96D6E0AC3C594A20 Content-Type: text/plain; charset=3Dutf-8; format=3Dflowed Content-Transfer-Encoding: 7bit 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 = > > 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-vo= lumes-and-quorum/ --------------B6FFA6AC96D6E0AC3C594A20 Content-Type: text/html; charset=3Dutf-8 Content-Transfer-Encoding: 8bit

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.fredia= ni(a)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.
=C2=A0

If I understand correctly this seems overkill and waste of storage as 2 copies of data (replica 2)=C2=A0 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/Administr= ator%20Guide/arbiter-volumes-and-quorum/

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