[jira] [Resolved] (HBASE-29707) Fix region cache % metrics miss calculation

["Wellington Chevreuil (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-29707?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Wellington Chevreuil resolved HBASE-29707.
------------------------------------------
    Resolution: Fixed

Merged into master, branch-3, branch-2 and branch-2.6. Thanks for the reviews, [~psomogyi] and [~kgeiszler]!

> Fix region cache % metrics miss calculation
> -------------------------------------------
>
>                 Key: HBASE-29707
>                 URL: https://issues.apache.org/jira/browse/HBASE-29707
>             Project: HBase
>          Issue Type: Bug
>          Components: BucketCache
>    Affects Versions: 3.0.0-beta-1, 2.7.0, 2.6.4
>            Reporter: Wellington Chevreuil
>            Assignee: Wellington Chevreuil
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 2.7.0, 3.0.0-beta-2, 2.6.5
>
>
> HBASE-28246 has introduced this metric that tracks the percentage of regions data that is cached, displaying this info on the RS UI store file metrics tab. 
> Unfortunately, under the following scenarios, this metric can be miscalculated and display wrong information:
> 1) Region compactions: During compactions with cacheCompactedOnWrite set to true, we cache the new blocks written during compaction, which correctly updates the related metric, but once compaction is finished and compacted files have its readers closed, we are evicting the blocks but missing to decrease those from the metric.
> 2) Cache of hlinks pointing to files in archive: When caching an HLink for a file in the archive folder from a region that's still online, we are adding that file cached size to the original region, which is wrong, since the archived file blocks are not relevant for the original region, but to the region containing the link.
> In both cases, affected regions will show the “% Cache” metric beyond 100% on the Web UI, which can be misleading. Also, this metric is used by the CacheAwareLoadBalancer, and over counting region cache percentage can impact this balancer efficiency.



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