leventov commented on issue #6993: [Proposal] Dynamic prioritization and laning
URL: 
https://github.com/apache/incubator-druid/issues/6993#issuecomment-534179078
 
 
   @sascha-coenen
   
   > Is it really true that tiers could be made "completely" unnecessary? We 
are using tiers for fail safety, having one tier running in one availability 
zone and a tier holding replicas running in another availability zone. Would 
k-safety and multi-AZ cases also be possible in a world without tiers?
   
   After #4773 is done (or even before that), it's pretty straightforward to 
implement the transparent lazy download of segments from the deep storage on 
Historicals upon they are first queried. This, in turn, opens up the technical 
possibility to implement spinning up Historical tiers which are exact clones of 
other tiers in a matter of minutes rather than dozens of minutes or hours.
   
   When you are able to clone the whole Historical tier in a few minutes, I 
think keeping a backup tier in a different AZ is not optimal in almost all 
cases except the rare ones with very stringent reliability requirements (this 
is not typical for Druid which is used in analytics workloads which are rarely 
*that* critical).

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