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From "Elliott Clark (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8370) Report data block cache hit rates apart from aggregate cache hit rates
Date Wed, 03 Jul 2013 23:55:21 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13699614#comment-13699614

Elliott Clark commented on HBASE-8370:

bq.I disagree that its not actionable. I would go fix the block cache in this case. 
My opinion would be that metrics should be useful for they Guys and Girls on the ops teams.
 If the metrics published aren't usable by them then most users are paying a cost for things
that only interest us devs.  Tracing, trace level logging and debuggers should provide enough
info to diagnose most of the block cache features you are talking about.

The path, of adding metrics for devs, is what first got us SchemaMetrics. Some devs were investigating
how and when to use multi level indexes in HFileV2.  Everyone paid the cost for those metrics
even after the issues had all be solved.  I'm really just trying to make sure we don't repeat
the same mistakes.

bq.I would agree that index block metrics are not needed or actionable if it is indeed the
case that we pin index blocks forever.
They are always the last to be evicted yes. https://github.com/apache/hbase/blob/trunk/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CachedBlock.java#L125

> Report data block cache hit rates apart from aggregate cache hit rates
> ----------------------------------------------------------------------
>                 Key: HBASE-8370
>                 URL: https://issues.apache.org/jira/browse/HBASE-8370
>             Project: HBase
>          Issue Type: Improvement
>          Components: metrics
>            Reporter: Varun Sharma
>            Assignee: Varun Sharma
>            Priority: Minor
> Attaching from mail to dev@hbase.apache.org
> I am wondering whether the HBase cachingHitRatio metrics that the region server UI shows,
can get me a break down by data blocks. I always see this number to be very high and that
could be exagerated by the fact that each lookup hits the index blocks and bloom filter blocks
in the block cache before retrieving the data block. This could be artificially bloating up
the cache hit ratio.
> Assuming the above is correct, do we already have a cache hit ratio for data blocks alone
which is more obscure ? If not, my sense is that it would be pretty valuable to add one.

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