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From "Erik Rozendaal (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-1996) Configure scanner buffer in bytes instead of number of rows
Date Tue, 29 Dec 2009 08:26:29 GMT

    [ https://issues.apache.org/jira/browse/HBASE-1996?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12795026#action_12795026
] 

Erik Rozendaal commented on HBASE-1996:
---------------------------------------

For 0.20.3 we could have a configuration parameter for HRegionServer for the scan buffer size.
The client will not be able to override this value. It is not optimal, but will work for us
and will prevent the current worst-case scan behavior when caching is set to 1 and rows are
very small. The scan buffer size could default to 0 (100% backwards compatible) or a more
useful value like 128-512 kB or so.

W.r.t. "at least N rows and M bytes" 

I don't think this is very useful (yet). I think it is more useful to be able to specify:

- scan buffer size (for performance)
- allow/disallow partial rows (to prevent OOMs with large rows)

Adding minimum number of rows and/or batch size in number of key values (0.21?) will make
the API harder to use and test.

> Configure scanner buffer in bytes instead of number of rows
> -----------------------------------------------------------
>
>                 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
>
>         Attachments: 1966.patch
>
>
> 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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