This is a multi-part message in MIME format. --------------E1421BB04F7A8066E3169AB1 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit 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@upx.com <mailto:fernando.frediani@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@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-volum... <https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/>
--------------E1421BB04F7A8066E3169AB1 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit <html> <head> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> </head> <body bgcolor="#FFFFFF" text="#000000"> <p>Ok, great, thanks for the clarification.</p> <p>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.</p> <p>Regards<br> Fernando<br> </p> <br> <div class="moz-cite-prefix">On 24/04/2017 11:35, Denis Chaplygin wrote:<br> </div> <blockquote cite="mid:CANVzE5nokh4MnXYfh8-oSQ8v0o1R4cKP0q1k5SKWPBqBHXiVqg@mail.gmail.com" type="cite"> <div dir="ltr">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.<br> </div> <div class="gmail_extra"><br> <div class="gmail_quote">On Mon, Apr 24, 2017 at 3:33 PM, FERNANDO FREDIANI <span dir="ltr"><<a moz-do-not-send="true" href="mailto:fernando.frediani@upx.com" target="_blank">fernando.frediani@upx.com</a>></span> wrote:<br> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div bgcolor="#FFFFFF" text="#000000"> <p>But then quorum doesn't replicate data 3 times, does it ?</p> <span class="HOEnZb"><font color="#888888"> <p>Fernando<br> </p> </font></span> <div> <div class="h5"> <br> <div class="m_-3736559709896304842moz-cite-prefix">On 24/04/2017 10:24, Denis Chaplygin wrote:<br> </div> <blockquote 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" href="mailto:fernando.frediani@upx.com" target="_blank">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="m_-3736559709896304842gmail-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-volum..." target="_blank">https://gluster.readthedocs.<wbr>io/en/latest/Administrator%<wbr>20Guide/arbiter-volumes-and-<wbr>quorum/</a><br> </div> </div> </blockquote> <br> </div> </div> </div> </blockquote> </div> <br> </div> </blockquote> <br> </body> </html> --------------E1421BB04F7A8066E3169AB1--