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From "amit.mor.mail@gmail.com" <amit.mor.m...@gmail.com>
Subject Help with Read Latency Metrics Interpretation
Date Tue, 21 May 2013 15:20:57 GMT
I am on EC2, with HBase 0.94.2 and I can't find an explanation to this
phenomena:
I'm having these metrics on my RegionServers:

blockCacheHitRatio=87%
fsReadLatencyHistogramMean=  5886103.47
fsReadLatencyHistogramMedian=6280445
fsReadLatencyHistogram75th= 28117916.5
fsReadLatencyHistogram95th= 53674180.05

Now, I am wondering, if 87% of my reads are cached, what could be the
reason for 75% of the reads falling on 28.1ms - that is, I expect that the
75th percentile reads would be from cache - and with cache performance, not
to mention the 95th percentile. As far as I am concerned, the mean and
median are great for our use case but the tails are too wide and thick.

Secondly, I've noticed that when we're running mapreduce jobs, the number
of requestsPerSecond and all fs* and *requestsCount are peaking - afaik the
mapper (running cascading-maple) just reads directly from HDFS and should
not have any influence on the RPC's (for example) to the RegionServer and
the associated metrics, what is going on with the RS metrics ?

Thanks
Amit

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