Hi Markus,
> "It is generally not recommended to set a replication factor of 3 if
> you have fewer than six nodes in a data center".
Actually you can create a cluster with 3 nodes and replication level 3.
But in this case if one of them would fail cluster become inconsistent.
In this way minimum reasonable nodes number is 4 for replication level 3.
In this case we can tolerate single node failure. But in this situation
each node would contain 3/4 of all data. This is not very good. Number 6
is recommended because in this case each node contain 1/2 of all data,
this is quite adequate overhead.

Typically Cassandra clusters don't have big replication level, typically
it is 3 (failure of any single node don't crush cluster) or 5 (failure
of any two nodes don't crush cluster).

For more details you should look to replication level calculator
<http://www.ecyrd.com/cassandracalculator/>.

--
Thanks,
Sergey

On 14/04/14 13:25, Markus Jais wrote:
> Hello,
>
> currently reading the "Practical Cassandra". In the section about
> replication factors the book says:
>
> "It is generally not recommended to set a replication factor of 3 if
> you have fewer than six nodes in a data center".
>
> Why is that? What problems would arise if I had a replication factor
> of 3 and only 5 nodes?
>
> Does that mean that for a replication of 4 I would need at least 8
> nodes and for a factor of 5 at least 10 nodes?
>
> Not saying that I would factor 5 andn 10 nodes, just curious about how
> this works.
>
> All the best,
>
> Markus

Attachment: signature.asc
Description: OpenPGP digital signature

Reply via email to