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From aaron morton <>
Subject Re: Sizing a Cassandra cluster
Date Thu, 24 Mar 2011 02:04:49 GMT
It really does depend on what your workload is like, and in the end will involve a certain
amount of fudge factor. provides some guidance. can be used to get a rough idea of the
memory requirements. Note that secondary indexes are also CF's with the same memory settings
as the parent. 

With RF3 you can lose afford to lose one replica for a key a token range and still be available
(Assuming Quorum CL). With RF 5 you can lose 2 replicas and still be available for the keys
in the range. 

I'm been careful to say "lose X replicas" because the other nodes in the cluster don't count
when considering an operation for a key. Two examples, 9 node cluster with RF3. If you lose
nodes 2 and 3 and they are replicas for node 1, Quorum operations on keys in the range for
node 1 will fail (ranges for 2 and 3 will be ok). If you lose nodes 2 and 5 Quorum operations
will succeed for all keys. 

RF 3 is reasonable starting point for some redundancy, RF 5 is more. After that it's Web Scale

Hope that helps
On 24 Mar 2011, at 04:04, Brian Fitzpatrick wrote:

> I'm going through the process of specing out the hardware for a
> Cassandra cluster. The relevant specs:
> - Support 460 operations/sec (50/50 read/write workload). Row size
> ranges from 4 to 8K.
> - Support 29 million objects for the first year
> - Support 365 GB storage for the first year, based on Cassandra tests
> (data + index + overhead * replication factor of 3)
> I'm looking for advice on the node size for this cluster, recommended
> RAM per node, and whether RF=3 seems to be a good choice for general
> availability and resistance to failure.
> I've looked at the YCSB benchmark paper and through the archives of
> this email list looking for pointers.  I haven't found any general
> guidelines on recommended cluster size to support X operations/sec
> with Y data size at RF factor of Z, that I could extrapolate from.
> Any and all recommendations appreciated.
> Thanks,
> Brian

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