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HA : Rack awarness with two racks

Hi

Can we consider that with Rack awarness and 2 racks we can support the failure of one rack?

Lot of blogs and articles are saying yes. But what about Journal nodes and Zookeeper ? I'll have one rack with 2 journal nodes or zookeepers. Losing this particular rack will get my cluster down. Am I missing something ?

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Accepted Solutions

Re: HA : Rack awarness with two racks

Guru

Hi Tim,

Rack awareness is related to how blocks are replicated amongst a cluster, so you won't loose any data (and improve locality) by setting it; It's not related to how masters are configured, but you can ensure by configuring them accordingly that you won't get your cluster down. Note that you possibly install more than 3 zk or journal nodes (has pros and cons though)

There's a nice 2-parts article on HCC: https://community.hortonworks.com/articles/43057/rack-awareness-1.html

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Re: HA : Rack awarness with two racks

Guru

Hi Tim,

Rack awareness is related to how blocks are replicated amongst a cluster, so you won't loose any data (and improve locality) by setting it; It's not related to how masters are configured, but you can ensure by configuring them accordingly that you won't get your cluster down. Note that you possibly install more than 3 zk or journal nodes (has pros and cons though)

There's a nice 2-parts article on HCC: https://community.hortonworks.com/articles/43057/rack-awareness-1.html

View solution in original post

Re: HA : Rack awarness with two racks

Hi @Laurent Edel

Thanks for your answer. So I understand that's only for not loosing data and not for HA. What happens when I loose 2 ZK or 2 JN ? can I still read/write from the cluster ? (suppose that I have 3 instances of each)

Re: HA : Rack awarness with two racks

Guru

that's correct; Indeed, if you have only one JN left you'll get into troubles with namenodes going down, same thing for zookeeper which must have a quorum

from zookeeper doc : Three ZooKeeper servers is the minimum recommended size for an ensemble

Re: HA : Rack awarness with two racks

@Laurent Edel Sure, I understand the risk when the NN goes down. What happens if the active NN is still up and I loose two JNs or two ZKs ?

Re: HA : Rack awarness with two racks

Guru

if you loose 2 ZK then NN will stay up but if it goes down the failover won't occur.

if you loose 2 JNs your NNs will go down.