Re: [ovirt-users] why replica 3

9 Sep 2015

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On 09/09/2015 10=3A14 AM=2C Richard Neuboeck wrote=3A
=3E On 04=2E09=2E15 10=3A02=2C Simone Tiraboschi wrote=3A
=3E=3E     Is there a reason why it has to be exactly replica 3=3F
=3E=3E
=3E=3E
=3E=3E To have a valid quorum having the system being able to decide witch=
 is
=3E=3E the right and safe copy avoiding an issue called split brain=2E
=3E=3E Under certain circumstances/issues =28network issue=2C hosts down or=

=3E=3E whatever could happen=29 the data on different replica could diverge=
=3A if
=3E=3E you have two and just two different hosts that claim each other
=3E that its
=3E=3E copy is the right one there is no way to automatically take the righ=
t
=3E=3E decision=2E Having three hosts and setting the quorum according to t=
hat
=3E=3E solves/mitigates the issue=2E
=3E
=3E Thanks for the explanation=2E I do understand the problem but since
=3E I=27m somewhat limited in my hardware options is there a way to
=3E override this requirement=3F Meaning if I change the checks for
=3E replica 3 in the installation scripts does something else fail on
=3E the way=3F
=3E
=3E In my case coherence checks would come from outside the storage and
=3E vm host setup and fencing would be applied appropriately=2E
=3E
=3E I would very much appreciate it if the particulars of the storage
=3E setup could be either selected from a list of possibilities or be
=3E ignored and just a warning be issued that this setup is not recommended=
=2E
=3E
=3E Thanks!
=3E Richard
=3E
=3E
As a side question=2C in Glusterfs 3=2E7 there is an =22AFR arbiter volume=
=22
=28http=3A//gluster=2Ereadthedocs=2Eorg/en/release-3=2E7=2E0-1/Features/afr=
-arbiter-volumes/=29
that will only contain the metadata=2E

Will ovirt support this in 3=2E6=3F

Kind regards=2C

Jorick Astrego

Met vriendelijke groet=2C With kind regards=2C

Jorick Astrego

Netbulae Virtualization Experts=20

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On 09/09/2015 10:14 AM, Richard Neuboeck wrote: 
> On 04.09.15 10:02, Simone Tiraboschi wrote:=
3; 
>>     Is there a reason =
why it has to be exactly replica 3? 
>> 
>> 
>> To have a valid quorum having the system b=
eing able to decide witch is 
>> the right and safe copy avoiding an issue =
called split brain. 
>> Under certain circumstances/issues (networ=
k issue, hosts down or 
>> whatever could happen) the data on differe=
nt replica could diverge: if 
>> you have two and just two different hosts =
that claim each other 
> that its 
>> copy is the right one there is no way to a=
utomatically take the right 
>> decision. Having three hosts and setting t=
he quorum according to that 
>> solves/mitigates the issue.<br=
...
> 
> Thanks for the explanation. I do understand th=
e problem but since 
> I'm somewhat limited in my hardware options is=
 there a way to 
> override this requirement? Meaning if I change=
 the checks for 
> replica 3 in the installation scripts does som=
ething else fail on 
> the way? 
> 
> In my case coherence checks would come from ou=
tside the storage and 
> vm host setup and fencing would be applied app=
ropriately. 
> 
> I would very much appreciate it if the particu=
lars of the storage 
> setup could be either selected from a list of =
possibilities or be 
> ignored and just a warning be issued that this=
 setup is not recommended. 
> 
> Thanks! 
> Richard 
> 
> 
As a side question, in Glusterfs 3.7 there is an "AFR arbiter volume&q=
uot; 
(<a href=3D"http://gluster.readthedocs.org/en/release-3.7.0-1/Features/afr-=
arbiter-volumes/">http://gluster.readthedocs.org/en/release-3.7.0-1/Feature=
s/afr-arbiter-volumes/</a>) 
that will only contain the metadata. 
 
Will ovirt support this in 3.6? 
 
Kind regards, 
 
Jorick Astrego 
=

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