[jira] [Updated] (CASSANDRA-17808) Optionally avoid hint transfer during decommission

["Caleb Rackliffe (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-17808?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Caleb Rackliffe updated CASSANDRA-17808:
----------------------------------------
    Test and Documentation Plan: A new test verifying the absence of hint transfer during decommission has been added.
                         Status: Patch Available  (was: In Progress)

[~stefan.miklosovic] Not sure if you were interested in reviewing further, but [the PR is ready|https://github.com/apache/cassandra/pull/1835].

> Optionally avoid hint transfer during decommission
> --------------------------------------------------
>
>                 Key: CASSANDRA-17808
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-17808
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Consistency/Hints
>            Reporter: Caleb Rackliffe
>            Assignee: Caleb Rackliffe
>            Priority: Normal
>             Fix For: 4.x
>
>          Time Spent: 40m
>  Remaining Estimate: 0h
>
> Both because they aren’t strictly necessary to maintain consistency, and because throttling induced by their rate-limiter (see {{hinted_handoff_throttle}}) may stall progress, transferring hints during decommission (specifically unbootstrap) rather than just pausing, disabling, and truncating them probably doesn’t make sense. The only other concern would be the BatchLog, which nominally depends on hint delivery to maintain its "guarantees". However, during BatchLog replay on unbootstrap, {{ReplayingBatch}} ignores batches older the gcgs anyway.
> Here's a proposal from [~aleksey] that might strike a reasonable balance:
> 1.) We continue to transfer hints by default during decommission, but at a higher rate. We could, for instance, stop having {{DispatchHintsTask}} divide its effective rate by the number of nodes in the cluster.
> {noformat}
> int nodesCount = Math.max(1, StorageService.instance.getTokenMetadata().getAllEndpoints().size() - 1);
> double throttleInBytes = DatabaseDescriptor.getHintedHandoffThrottleInKiB() * 1024.0 / nodesCount;
> this.rateLimiter = RateLimiter.create(throttleInBytes == 0 ? Double.MAX_VALUE : throttleInBytes);
> {noformat}
> 2.) We provide an option to simply avoid transferring hints during unbootstrap. Even this would only take the BatchLog from "best effort" to "slightly less effort" ;)



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