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https://issues.apache.org/jira/browse/HBASE-8370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13694336#comment-13694336
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Varun Sharma commented on HBASE-8370:
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"We don't have per block type metrics in trunk/95 because the overall cache hit
percentage is a good proxy for data block cache percent. Yes the overall number
is higher but it still gives a good actionable number. You can know if you're
doing better or worse than you were before. Even better is the derivative of
cache miss count."
I am not sure this is true - this number is always 99 % for us on all clusters
- blockCacheHitCachingRation - how can a number which never changes, ever be
actionable ? Even with decimal numbers, its never going to change because the
index blocks are going to take over
Also, the different b/w 82 % cache hit ratio to 99 % cache hit ratio is
enormous. Controlling you p80 on latency is a *lot* easier than your p99. A
cache hit ratio of 99 % just sends you this false sense of security that you
have controlled your p99 latency. This is important for online serving, maynot
be for enterprise.
I guess, we don't need to bring back SchemaMetrics to fix this but we can have
block level metrics. At least I want to be sure that Index blocks have 100 %
cache hit rates because if that's not happening, then I am in a bad situation.
It would be better to not have folks using HBase for online storage, play a
guessing game, as to what the true effectiveness of the cache is.
> 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 [email protected]
> 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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