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From Robert Coli <>
Subject Re: Changing replication factor
Date Sat, 15 Jun 2013 00:20:26 GMT
On Mon, Jun 10, 2013 at 6:04 AM, Vegard  Berget <> wrote:
> If one increases the replication factor of a keyspace and then do a repair,
> how will this affect the performance of the affected nodes? Could we risk
> the nodes being (more or less) unresponsive while repair is going on?

Repair is a relatively heavyweight activity (the heaviest a cassandra
node can do!) which requires significant headroom in terms of CPU,
heap memory and disk space. It is possible that nodes could become
unavailable transiently during the repair, but unless they are already
very busy they should not become completely unresponsive. For one
thing, both compaction and streaming respect throttles which are
designed to minimize the impact of the streaming/compaction workload
resulting from repair.

>  The nodes I am speaking of contains ~100gb of data.

This is a relatively small amount of data per node, which makes the
impact of Repair less severe.

> Also, some of the keyspaces I am considering increase the replication factor
> for contains Counter Column Families (has rf:1).  I think I have read that
> adding replication to counter cfs will affect performance negatively, is
> this correct?

Per Sylvain (one of the primary authors of the Counters codebase)  [1] :

For counters, it's a little bit different. At RF=3, for each inserts,
one node is doing a write *and* a read, while the two other nodes are
only doing a
write. So given that the read takes a time is non negligible, you
should see simple
improvement a RF=3 compared to RF=1 because each node gets 1/3 of the
reads (involved in
the counter write) it would get if it was the only replica. Now if the
write time
were negligible compared to the read time, then yes you would see roughly a 3x
increase. But while writes are still faster than reads in Cassandra,
reads a now fairly
fast too (but all this depends on other factor like how much the
caches helps, etc...), so it
will likely be less than a 3x increase. Should be noticeable though."

I interpret the above to mean that RF=3 is actually slightly *faster*
for Counters than RF=1.



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