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From Eric Stevens <>
Subject Re: Re[2]: Redundancy inside a cassandra node
Date Sat, 08 Nov 2014 14:24:29 GMT
> They do not use Raid10 on the node, they don't use dual power as well,
because it's not cheap in cluster of many nodes

I think the point here is that money spent on traditional failure avoidance
models is better spent in a Cassandra cluster by instead having more nodes
of less expensive hardware.  Rather than redundant disks network ports and
power supplies, spend that money on another set of nodes in a different
topological (and probably physical) rack.  The parallel to having redundant
disk arrays is to increase replication factor (RF=3 is already one replica
better than Raid 10, and with fewer SPOFs).

The only reason I can think you'd want to double down on hardware failover
like the traditional model is if you are constrained in your data center
(eg, space or cooling) and you'd rather run machines which are individually
physically more resilient in exchange for running a lower RF.

On Sat Nov 08 2014 at 5:32:22 AM Plotnik, Alexey <> wrote:

>  Let me speak from my heart. I maintenance 200+TB Cassandra cluster. The
> problem is money. If your IT people have a $$$ they can deploy Cassandra on
> super robust hardware with triple power supply of course. But why then you
> need Cassandra? Only for scalability?
> The idea of high available clusters is to get robustness from availability
> (not from hardware reliability). More availability (more nodes) you have -
> more money you need to buy hardware. Cassandra is the most high available
> system on the planet - it scaled horizontally to any number of nodes. You
> have time series data, you can set replication factor > 3 if needed.
> There is a concept of network topology in Cassandra - you can specify on
> which *failure domain* (racks or independent power lines) your nodes
> installed on, and then replication will be computed correspondingly to
> store replicas of a specified data on a different failure domains. The same
> is for DC - there is a concept of data center in Cassandra topology, it
> knows about your data centers.
> You should think not about hardware but about your data model - is
> Cassandra applicable for you domain? Thinks about queries to your
> data. Cassandra is actually a key value storage (documentation says it's a
> column based storage, but it's just an CQL-abstraction over key and binary
> value, nothing special except counters) so be very careful in designing
> your data model.
> Anyway, let me answer your original question:
> > what do people use in the real world in terms of node resiliancy when
> running a cassandra cluster?
> Nothing because Cassandra is high available system. They use SSDs if they
> need speed. They do not use Raid10 on the node, they don't use dual power
> as well, because it's not cheap in cluster of many nodes and have no sense
> because reliability is ensured by replication in large clusters. Not sure
> about dual NICs, network reliability is ensured by distributing your
> cluster across multiple data centers.
> We're using single SSD and single HDD on each node (we symlink some CF
> folders to other disk). SSD for CFs where we need low latency, HDD for
> binary data. If one of them fails, replication save us and we have
> time to deploy new node and load data from replicas with Cassandra repair
> feature back to original node. And we have no problem with it, node fail
> sometimes, but it doesn't affect customers. That is.
> ------ Original Message ------
> From: "Jabbar Azam" <>
> To: "" <>
> Sent: 08.11.2014 19:43:18
> Subject: Re: Redundancy inside a cassandra node
> Hello Alexey,
> The node count is 20 per site and there will be two sites. RF=3. But since
> the software isn't complete and the database code is going through a
> rewrite we aren't sure about space requirements. The node count is only a
> guess, bases on the number of dev nodes in use. We will have better
> information when the rewrite is done and testing resumes.
> The data will be time series data. It was binary blobs originally but we
> have found that the new datastax c# drivers have improved alot in terms of
> read performance.
> I'm curious. What is your definition of commodity. My IT people seem to
> think that the servers must be super robust. Personally I'm not sure if
> that should be the case.
> The node
>  Thanks
> Jabbar Azam
> On 8 November 2014 02:56, Plotnik, Alexey <> wrote:
>> Cassandra is a cluster itself, it's not necessary to have redundant each
>> node. Cassandra has replication for that. And also Cassandra is designed to
>> run in multiple data center - am think that redundant policy is applicable
>> for you. Only thing from your saying you can deploy is raid10, other don't
>> make any sense. As you are in stage of designing you cluster, please
>> provide some numbers: how many data will be stored on each node, how many
>> nodes would you have? What type of data will be stored in cluster: binary
>> object o something time series?
>> Cassandra is designed to run on commodity hardware.
>> Отправлено с iPad
>> > 8 нояб. 2014 г., в 6:26, Jabbar Azam <> написал(а):
>>  >
>> > Hello all,
>> >
>> > My work will be deploying a cassandra cluster next year. Due to
>> internal wrangling we can't seem to agree on the hardware. The software
>> hasn't been finished, but management are asking for a ballpark figure for
>> the hardware costs.
>> >
>> > The problem is the IT team are saying the nodes need to have multiple
>> points of redundancy
>> >
>> > e.g. dual power supplies, dual nics, SSD's configured in raid 10.
>> >
>> >
>> > The software team is saying that due to cassandras resilient nature,
>> due to the way data is distributed and scalability that lots of cheap boes
>> should be used. So they have been taling about self build consumer grade
>> boxes with single nics, PSU's single SSDs etc.
>> >
>> > Obviously the self build boxes will cost a fraction of the price, but
>> each box is not as resilient as the first option.
>> >
>> > We don;t use any cloud technologies, so that's out of the question.
>> >
>> > My question is what do people use in the real world in terms of node
>> resiliancy when running a cassandra cluster?
>> >
>> > Write now the team is only thinking of hosting cassandra on the nodes.
>> I'll see if I can twist their arms and see the light with Apache Spark.
>> >
>> > Obviously there are other tiers of servers, but they won't be running
>> cassandra.
>> >
>> >
>> >
>> >
>> >
>> > Thanks
>> >
>> > Jabbar Azam

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