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From "Dave Latham (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-1996) Configure scanner buffer in bytes instead of number of rows
Date Sat, 21 Nov 2009 00:11:39 GMT
Configure scanner buffer in bytes instead of number of rows
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                 Key: HBASE-1996
                 URL: https://issues.apache.org/jira/browse/HBASE-1996
             Project: Hadoop HBase
          Issue Type: Improvement
            Reporter: Dave Latham
            Assignee: Dave Latham
             Fix For: 0.21.0


Currently, the default scanner fetches a single row at a time.  This makes for very slow scans
on tables where the rows are not large.  You can change the setting for an HTable instance
or for each Scan.

It would be better to have a default that performs reasonably well so that people stop running
into slow scans because they are evaluating HBase, aren't familiar with the setting, or simply
forgot.  Unfortunately, if we increase the value of the current setting, then we run the risk
of running OOM for tables with large rows.  Let's change the setting so that it works with
a size in bytes, rather than in rows.  This will allow us to set a reasonable default so that
tables with small rows will scan performantly and tables with large rows will not run OOM.

Note that the case is very similar to table writes as well.  When disabling auto flush, we
buffer a list of Put's to commit at once.  That buffer is measured in bytes, so that a small
number of large Puts or a lot of small Puts can each fit in a single flush.  If that buffer
were measured in number of Put's it would have the same problem that we have for the scan
buffer, and we wouldn't be able to set a good default value for tables with different size
rows.  Changing the scan buffer to be configured like the write buffer will make it more consistent.

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