[jira] [Created] (HBASE-27224) HFile tool statistic sampling produces misleading results

["Bryan Beaudreault (Jira)" <ji...@apache.org>]

Bryan Beaudreault created HBASE-27224:
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             Summary: HFile tool statistic sampling produces misleading results
                 Key: HBASE-27224
                 URL: https://issues.apache.org/jira/browse/HBASE-27224
             Project: HBase
          Issue Type: Improvement
            Reporter: Bryan Beaudreault


HFile tool uses codahale metrics for collecting statistics about key/values in an HFile. We recently had a case where the statistics printed out that the max row size was only 25k. This was confusing because I was seeing bucket cache allocation failures for blocks as large as 1.5mb. 

Digging in, I was able to find the large row using the "-p" argument (which was obviously very verbose). Once I found the row, I saw the vlen was listed as ~1.5mb which made much more sense.

First thing I notice here is that default codahale metrics histogram is using ExponentiallyDecayingReservoir. This probably makes sense for a long-lived histogram, but the HFile tool is run at a point in time. It might be best to use UniformReservoir instead.

Secondly, we do not need sampling for min/max. Let's supplement the histogram with our own calculation which is guaranteed to be accurate for the entirety of the file.



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