9:33 a.m.
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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(a)upx.com <mailto:fernando.frediani@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-vo...
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<p>But then quorum doesn't replicate data 3 times, does it ?</p>
<p>Fernando<br>
</p>
<br>
<div class="moz-cite-prefix">On 24/04/2017 10:24, Denis Chaplygin
wrote:<br>
</div>
<blockquote
cite="mid:CANVzE5kfHSF15iwRYxagU-rSEvN60XSZcjc8jVSTrKdYuXeeEg@mail.gmail.com"
type="cite">
<div dir="ltr">Hello!<br>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Apr 24, 2017 at 3:02 PM,
FERNANDO FREDIANI <span dir="ltr"><<a
moz-do-not-send="true" target="_blank"
href="mailto:fernando.frediani@upx.com">fernando.frediani@upx.com</a>></span>
wrote:<br>
<blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px
solid rgb(204,204,204);padding-left:1ex"
class="gmail_quote">
<div bgcolor="#FFFFFF">Out of curiosity, why do you and
people in general use more replica 3 than replica 2 ? </div>
</blockquote>
<div><br>
</div>
<div>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.<br>
</div>
<div> </div>
<blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px
solid rgb(204,204,204);padding-left:1ex"
class="gmail_quote">
<div bgcolor="#FFFFFF">
<p>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.</p>
<span class="gmail-HOEnZb"></span><br>
</div>
</blockquote>
</div>
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]. <br>
<br>
[1] <a moz-do-not-send="true"
href="https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/">https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/</a><br>
</div>
</div>
</blockquote>
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