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Date: Tue, 5 Aug 2014 23:27:58 +0200
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 <CA+LRppgfQrWUJoaAg3yjQpqUV_VauruVn=AukZ1w0xcrUA3-_w@mail.gmail.com>
Subject: Re: Make an existing cluster multi data-center compatible.
From: Rene Kochen <rene.kochen@schange.com>
To: user@cassandra.apache.org
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"As long as you correctly configure the new snitch so that the replica sets
do not change, no, you do not need to repair."

Is the following correct:

The replica sets do not change if you modify the snitch from SimpleSnitch
to NetworkTopologyStrategy and the topology file puts all nodes in the same
data-center and rack.

Thanks again!

Rene


2014-08-05 20:05 GMT+02:00 Robert Coli <rcoli@eventbrite.com>:

> On Tue, Aug 5, 2014 at 3:52 AM, Rene Kochen <rene.kochen@schange.com>
> wrote:
>
>> Do I have to run full repairs after this change? Because the yaml file
>> states: IF YOU CHANGE THE SNITCH AFTER DATA IS INSERTED INTO THE CLUSTER,
>> YOU MUST RUN A FULL REPAIR, SINCE THE SNITCH AFFECTS WHERE REPLICAS ARE
>> PLACED.
>>
>
> As long as you correctly configure the new snitch so that the replica sets
> do not change, no, you do not need to repair.
>
> Barring that, if you manage to transform the replica set in such a way
> that you always have one (fully repaired) replica from the old set, repair
> will help. I do not recommend this very risky practice. In practice the
> only transformation of snitch in a cluster with data which is likely to be
> safe is one whose result is a NOOP in terms of replica placement.
>
> In fact, the yaml file is stating something unreasonable there, because
> repair cannot protect against this case :
>
> - 6 node cluster, A B C D E F,  RF = 2
>
> 1) Start with SimpleSnitch so that A, B have the two replicas of row key X.
> 2) Write row key X, value Y, to nodes A and B.
> 2) Change to OtherSnitch so that now C,D are responsible for row key X.
> 3) Repair and notice that neither C nor D answer "Y" when asked for row X.
>
> =Rob
>
>

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<div dir=3D"ltr">&quot;As long as you correctly configure the new snitch so=
 that the replica sets do not change, no, you do not need to repair.&quot;<=
br><br>Is the following correct:<br><br>The replica sets do not change if y=
ou modify the snitch from SimpleSnitch to NetworkTopologyStrategy and the t=
opology file puts all nodes in the same data-center and rack.<br>

<br>Thanks again!<br><br>Rene<br></div><div class=3D"gmail_extra"><br><br><=
div class=3D"gmail_quote">2014-08-05 20:05 GMT+02:00 Robert Coli <span dir=
=3D"ltr">&lt;<a href=3D"mailto:rcoli@eventbrite.com" target=3D"_blank">rcol=
i@eventbrite.com</a>&gt;</span>:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_extra">=
<div class=3D"gmail_quote">On Tue, Aug 5, 2014 at 3:52 AM, Rene Kochen <spa=
n dir=3D"ltr">&lt;<a href=3D"mailto:rene.kochen@schange.com" target=3D"_bla=
nk">rene.kochen@schange.com</a>&gt;</span> wrote:<br>

<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr">Do I have to run full repai=
rs after this change? Because the yaml file states: IF YOU CHANGE THE SNITC=
H AFTER DATA IS INSERTED INTO THE CLUSTER, YOU MUST RUN A FULL REPAIR, SINC=
E THE SNITCH AFFECTS WHERE REPLICAS ARE PLACED.<br>

</div></blockquote><div><br></div><div>As long as you correctly configure t=
he new snitch so that the replica sets do not change, no, you do not need t=
o repair.</div><div><br></div><div>Barring that, if you manage to transform=
 the replica set in such a way that you always have one (fully repaired) re=
plica from the old set, repair will help. I do not recommend this very risk=
y practice. In practice the only transformation of snitch in a cluster with=
 data which is likely to be safe is one whose result is a NOOP in terms of =
replica placement.</div>

<div><br></div><div>In fact, the yaml file is stating something unreasonabl=
e there, because repair cannot protect against this case :</div><div>=C2=A0=
</div><div>- 6 node cluster, A B C D E F, =C2=A0RF =3D 2</div><div><br></di=
v><div>
1) Start with SimpleSnitch so that A, B have the two replicas of row key X.=
</div>
<div>2) Write row key X, value Y, to nodes A and B.</div><div>2) Change to =
OtherSnitch so that now C,D are responsible for row key X.</div><div>3) Rep=
air and notice that neither C nor D answer &quot;Y&quot; when asked for row=
 X.</div>

<div><br></div><div>=3DRob</div><div><br></div></div></div></div>
</blockquote></div><br></div>

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