<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Sep 9, 2015 at 10:14 AM, Richard Neuboeck <span dir="ltr"><<a href="mailto:hawk@tbi.univie.ac.at" target="_blank">hawk@tbi.univie.ac.at</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span>On 04.09.15 10:02, Simone Tiraboschi wrote:<br>
</span><span>> Is there a reason why it has to be exactly replica 3?<br>
><br>
><br>
> To have a valid quorum having the system being able to decide witch is<br>
> the right and safe copy avoiding an issue called split brain.<br>
> Under certain circumstances/issues (network issue, hosts down or<br>
> whatever could happen) the data on different replica could diverge: if<br>
> you have two and just two different hosts that claim each other<br>
that its<br>
> copy is the right one there is no way to automatically take the right<br>
> decision. Having three hosts and setting the quorum according to that<br>
> solves/mitigates the issue.<br>
<br>
<br>
</span>Thanks for the explanation. I do understand the problem but since<br>
I'm somewhat limited in my hardware options is there a way to<br>
<span>override this requirement? Meaning if I change the checks for<br>
replica 3 in the installation scripts does something else fail on<br>
the way?<br></span></blockquote><div><br></div>I'm advising that it's not a safe configuration so it's not recommended for a production environment.<br>Having said that, as far as I know it's enforced only in the setup script so tweaking it should be enough.</div><div class="gmail_quote">Otherwise, if you have enough disk space, you can also have a different trick: you could create a replica 3 volume with 2 bricks from a single host.</div><div class="gmail_quote">It's not a safe procedure at all cause you still have only 2 hosts, so it's basically just replica 2, and in case of split brain the host with two copies will win by configuration which is not always the right decision.</div><div class="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span>
</span>In my case coherence checks would come from outside the storage and<br>
vm host setup and fencing would be applied appropriately.<br></blockquote><div><br></div><div>Can I ask how?</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
I would very much appreciate it if the particulars of the storage<br>
setup could be either selected from a list of possibilities or be<br>
ignored and just a warning be issued that this setup is not recommended.<br>
<br>
Thanks!<br>
<span><font color="#888888">Richard<br>
<br>
<br>
--<br>
/dev/null<br>
<br>
</font></span></blockquote></div><br></div></div>