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From "Erik Rozendaal (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HBASE-1996) Configure scanner buffer in bytes instead of number of rows
Date Thu, 31 Dec 2009 10:14:29 GMT

     [ https://issues.apache.org/jira/browse/HBASE-1996?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Erik Rozendaal updated HBASE-1996:
----------------------------------

    Attachment: 1996-0.20.3-v2.patch

Second version of patch for 0.20.3 branch. This makes the maximum result size configurable.

However: the client and the server *must* use the same maximum result size, otherwise rows
in regions may be skipped. This is because of the way the results of a region scan are reported
to the client:

- null: scanning filter stopped processing
- fewer rows returned than requested: end-of-region reached, move on.

The second point is why the HTable modifications are necessary. It is now normal that a region
scan will return fewer rows than requested even when the end of the region has not been reached
yet. So the client needs to duplicate the region server logic to keep in sync.

I think for 0.21 the result communication to the client should be made more explicit, eg.
make a ScannerCallableResult class that contains a status field (MORE_AVAILABLE, SKIP_TO_NEXT_REGION,
FILTER_SAID_STOP) as well as the actual rows returned.

I also left the default max result size value at 1 megabyte. In my (admittedly limited) testing
using just my laptop without a real network a size of 256-1024 kB seems to be optimal.

Here are my test results:

||max scanner result size (bytes)||MB/s scanned with rows avg 750 bytes||MB/s scanned with
rows avg 175 bytes||
|1024|3.23|1.99|
|2048|5.14|3.10|
|4096|7.34|4.67|
|8192|10.95|6.50|
|16384|16.15|8.30|
|32768|18.96|8.50|
|65536|20.42|9.16|
|131072|20.93|9.06|
|262144|21.48|9.49|
|524288|22.34|9.37|
|1048576|22.50|8.91|
|2097152|20.91|8.03|
|4194304|19.86|7.35|
|8388608|17.89|6.83|
|16777216|17.63|6.98|

Scanner caching was set to Integer.MAX_VALUE (unlimited number of rows). MB/s are measured
going through a web server, so raw HBase speed is probably double or higher. Obviously a real
cluster test should be done to measure real performance and otherwise tune the max result
size.


> 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, 1996-0.20.3-v2.patch, 1996-0.20.3.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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