Thanks Alexander!
I'm not a MS in math too) Unfortunately. Not sure, but it seems to me that probability of 2/49 in your explanation doesn't take into account that vnodes endpoints are almost evenly distributed across all nodes (al least it's what I can see from "nodetool ring" output). http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureDataDistributeDistribute_c.html of course this vnodes illustration is a theoretical one, but there no 2 nodes on that diagram that can be switched off without losing a key range (at CL=QUORUM). That's because vnodes_per_node=8 > Nnodes=6. As far as I understand, situation is getting worse with increase of vnodes_per_node/Nnode ratio. Please, correct me if I'm wrong. How would the situation differ from this example by DataStax, if we had a real-life 6-nodes cluster with 8 vnodes on each node? Regards, Kyrill ________________________________ From: Alexander Dejanovski <a...@thelastpickle.com> Sent: Monday, January 15, 2018 8:14:21 PM To: user@cassandra.apache.org Subject: Re: vnodes: high availability I was corrected off list that the odds of losing data when 2 nodes are down isn't dependent on the number of vnodes, but only on the number of nodes. The more vnodes, the smaller the chunks of data you may lose, and vice versa. I officially suck at statistics, as expected :) Le lun. 15 janv. 2018 à 17:55, Alexander Dejanovski <a...@thelastpickle.com<mailto:a...@thelastpickle.com>> a écrit : Hi Kyrylo, the situation is a bit more nuanced than shown by the Datastax diagram, which is fairly theoretical. If you're using SimpleStrategy, there is no rack awareness. Since vnode distribution is purely random, and the replica for a vnode will be placed on the node that owns the next vnode in token order (yeah, that's not easy to formulate), you end up with statistics only. I kinda suck at maths but I'm going to risk making a fool of myself :) The odds for one vnode to be replicated on another node are, in your case, 2/49 (out of 49 remaining nodes, 2 replicas need to be placed). Given you have 256 vnodes, the odds for at least one vnode of a single node to exist on another one is 256*(2/49) = 10.4% Since the relationship is bi-directional (there are the same odds for node B to have a vnode replicated on node A than the opposite), that doubles the odds of 2 nodes being both replica for at least one vnode : 20.8%. Having a smaller number of vnodes will decrease the odds, just as having more nodes in the cluster. (now once again, I hope my maths aren't fully wrong, I'm pretty rusty in that area...) How many queries that will affect is a different question as it depends on which partition currently exist and are queried in the unavailable token ranges. Then you have rack awareness that comes with NetworkTopologyStrategy : If the number of replicas (3 in your case) is proportional to the number of racks, Cassandra will spread replicas in different ones. In that situation, you can theoretically lose as many nodes as you want in a single rack, you will still have two other replicas available to satisfy quorum in the remaining racks. If you start losing nodes in different racks, we're back to doing maths (but the odds will get slightly different). That makes maintenance predictable because you can shut down as many nodes as you want in a single rack without losing QUORUM. Feel free to correct my numbers if I'm wrong. Cheers, On Mon, Jan 15, 2018 at 5:27 PM Kyrylo Lebediev <kyrylo_lebed...@epam.com<mailto:kyrylo_lebed...@epam.com>> wrote: Thanks, Rahul. But in your example, at the same time loss of Node3 and Node6 leads to loss of ranges N, C, J at consistency level QUORUM. As far as I understand in case vnodes > N_nodes_in_cluster and endpoint_snitch=SimpleSnitch, since: 1) "secondary" replicas are placed on two nodes 'next' to the node responsible for a range (in case of RF=3) 2) there are a lot of vnodes on each node 3) ranges are evenly distribusted between vnodes in case of SimpleSnitch, we get all physical nodes (servers) having mutually adjacent token rages. Is it correct? At least in case of my real-world ~50-nodes cluster with nvodes=256, RF=3 for this command: nodetool ring | grep '^<ip-prefix>' | awk '{print $1}' | uniq | grep -B2 -A2 '<ip_of_a_node>' | grep -v '<ip_of_a_node>' | grep -v '^--' | sort | uniq | wc -l returned number which equals to Nnodes -1, what means that I can't switch off 2 nodes at the same time w/o losing of some keyrange for CL=QUORUM. Thanks, Kyrill ________________________________ From: Rahul Neelakantan <ra...@rahul.be<mailto:ra...@rahul.be>> Sent: Monday, January 15, 2018 5:20:20 PM To: user@cassandra.apache.org<mailto:user@cassandra.apache.org> Subject: Re: vnodes: high availability Not necessarily. It depends on how the token ranges for the vNodes are assigned to them. For example take a look at this diagram http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureDataDistributeDistribute_c.html In the vNode part of the diagram, you will see that Loss of Node 3 and Node 6, will still not have any effect on Token Range A. But yes if you lose two nodes that both have Token Range A assigned to them (Say Node 1 and Node 2), you will have unavailability with your specified configuration. You can sort of circumvent this by using the DataStax Java Driver and having the client recognize a degraded cluster and operate temporarily in downgraded consistency mode http://docs.datastax.com/en/latest-java-driver-api/com/datastax/driver/core/policies/DowngradingConsistencyRetryPolicy.html - Rahul On Mon, Jan 15, 2018 at 10:04 AM, Kyrylo Lebediev <kyrylo_lebed...@epam.com<mailto:kyrylo_lebed...@epam.com>> wrote: Hi, Let's say we have a C* cluster with following parameters: - 50 nodes in the cluster - RF=3 - vnodes=256 per node - CL for some queries = QUORUM - endpoint_snitch = SimpleSnitch Is it correct that 2 any nodes down will cause unavailability of a keyrange at CL=QUORUM? Regards, Kyrill -- ----------------- Alexander Dejanovski France @alexanderdeja Consultant Apache Cassandra Consulting http://www.thelastpickle.com<http://www.thelastpickle.com/> -- ----------------- Alexander Dejanovski France @alexanderdeja Consultant Apache Cassandra Consulting http://www.thelastpickle.com<http://www.thelastpickle.com/>
