No, that’s not correct. The point of racks is to help you distribute the replicas, not further-replicate the replicas. Data centers are what do the latter. So for example, if you wanted to be able to ensure that you always had quorum if an AZ went down, then you could have two DCs where one was in each AZ, and use one rack in each DC. In your situation I think I’d be more tempted to consider that. Then if an AZ went away, you could fail over your traffic to the remaining DC and still be perfectly fine.
For background on replicas vs racks, I believe the information you want is under the heading ‘NetworkTopologyStrategy’ at: http://cassandra.apache.org/doc/latest/architecture/dynamo.html That should help you better understand how replicas distribute. As mentioned before, while you can choose to do the reads in one DC, except for concerns about contention related to network traffic and connection handling, you can’t isolate reads from writes. You can _mostly_ insulate the write DC from the activity within the read DC, and even that isn’t an absolute because of repairs. However, your mileage may vary, so do what makes sense for your usage pattern. R From: Sergio <lapostadiser...@gmail.com> Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org> Date: Wednesday, October 23, 2019 at 12:50 PM To: "user@cassandra.apache.org" <user@cassandra.apache.org> Subject: Re: Cassandra Rack - Datacenter Load Balancing relations Message from External Sender Hi Reid, Thanks for your reply. I really appreciate your explanation. We are in AWS and we are using right now 2 Availability Zone and not 3. We found our cluster really unbalanced because the keyspace has a replication factor = 3 and the number of racks is 2 with 2 datacenters. We want the writes spread across all the nodes but we wanted the reads isolated from the writes to keep the load on that node low and to be able to identify problems in the consumers (reads) or producers (writes) applications. It looks like that each rack contains an entire copy of the data so this would lead to replicate for each rack and then for each node the information. If I am correct if we have a keyspace with 100GB and Replication Factor = 3 and RACKS = 3 => 100 * 3 * 3 = 900GB If I had only one rack across 2 or even 3 availability zone I would save in space and I would have 300GB only. Please correct me if I am wrong. Best, Sergio Il giorno mer 23 ott 2019 alle ore 09:21 Reid Pinchback <rpinchb...@tripadvisor.com<mailto:rpinchb...@tripadvisor.com>> ha scritto: Datacenters and racks are different concepts. While they don't have to be associated with their historical meanings, the historical meanings probably provide a helpful model for understanding what you want from them. When companies own their own physical servers and have them housed somewhere, the questions arise on where you want to locate any particular server. It's a balancing act on things like network speed of related servers being able to talk to each other, versus fault-tolerance of having many servers not all exposed to the same risks. "Same rack" in that physical world tended to mean something like "all behind the same network switch and all sharing the same power bus". The morning after an electrical glitch fries a power bus and thus everything in that rack, you realize you wished you didn't have so many of the same type of server together. Well, they were servers. Now they are door stops. Badness and sadness. That's kind of the mindset to have in mind with racks in Cassandra. It's an artifact for you to separate servers into pools so that the disparate pools have hopefully somewhat independent infrastructure risks. However, all those servers are still doing the same kind of work, are the same version, etc. Datacenters are amalgams of those racks, and how similar or different they are from each other depends on what you want to do with them. What is true is that if you have N datacenters, each one of them must have enough disk storage to house all the data. The actual physical footprint of that data in each DC depends on the replication factors in play. Note that you sorta can't have "one datacenter for writes" because the writes will replicate across the data centers. You could definitely choose to have only one that takes read queries, but best to think of writing as being universal. One scenario you can have is where the DC not taking live traffic read queries is the one you use for maintenance or performance testing or version upgrades. One rack makes your life easier if you don't have a reason for multiple racks. It depends on the environment you deploy into and your fault tolerance goals. If you were in AWS and wanting to spread risk across availability zones, then you would likely have as many racks as AZs you choose to be in, because that's really the point of using multiple AZs. R On 10/23/19, 4:06 AM, "Sergio Bilello" <lapostadiser...@gmail.com<mailto:lapostadiser...@gmail.com>> wrote: Message from External Sender Hello guys! I was reading about https://urldefense.proofpoint.com/v2/url?u=https-3A__cassandra.apache.org_doc_latest_architecture_dynamo.html-23networktopologystrategy&d=DwIBaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=xmgs1uQTlmvCtIoGJKHbByZZ6aDFzS5hDQzChDPCfFA&s=9ZDWAK6pstkCQfdbwLNsB-ZGsK64RwXSXfAkOWtmkq4&e= I would like to understand a concept related to the node load balancing. I know that Jon recommends Vnodes = 4 but right now I found a cluster with vnodes = 256 replication factor = 3 and 2 racks. This is unbalanced because the racks are not a multiplier of the replication factor. However, my plan is to move all the nodes in a single rack to eventually scale up and down the node in the cluster once at the time. If I had 3 racks and I would like to keep the things balanced I should scale up 3 nodes at the time one for each rack. If I would have 3 racks, should I have also 3 different datacenters so one datacenter for each rack? Can I have 2 datacenters and 3 racks? If this is possible one datacenter would have more nodes than the others? Could it be a problem? I am thinking to split my cluster in one datacenter for reads and one for writes and keep all the nodes in the same rack so I can scale up once node at the time. Please correct me if I am wrong Thanks, Sergio --------------------------------------------------------------------- To unsubscribe, e-mail: user-unsubscr...@cassandra.apache.org<mailto:user-unsubscr...@cassandra.apache.org> For additional commands, e-mail: user-h...@cassandra.apache.org<mailto:user-h...@cassandra.apache.org>
