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https://issues.apache.org/jira/browse/HDFS-4817?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13660921#comment-13660921
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Colin Patrick McCabe commented on HDFS-4817:
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That's a good idea. {{CachingPolicy}} could be extended in the future to have a
lot of those features. It is sent over the wire using protobufs, so we could
easily add more fields in the future. In order to make it more similar to the
{{fadvise}} interface, maybe I should rename dropBehind to {{dontNeed}}
(similar to {{FADV_DONTNEED}})?
> make HDFS advisory caching configurable on a per-file basis
> -----------------------------------------------------------
>
> Key: HDFS-4817
> URL: https://issues.apache.org/jira/browse/HDFS-4817
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs-client
> Affects Versions: 3.0.0
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Priority: Minor
> Attachments: HDFS-4817.001.patch
>
>
> HADOOP-7753 and related JIRAs introduced some performance optimizations for
> the DataNode. One of them was readahead. When readahead is enabled, the
> DataNode starts reading the next bytes it thinks it will need in the block
> file, before the client requests them. This helps hide the latency of
> rotational media and send larger reads down to the device. Another
> optimization was "drop-behind." Using this optimization, we could remove
> files from the Linux page cache after they were no longer needed.
> Using {{dfs.datanode.drop.cache.behind.writes}} and
> {{dfs.datanode.drop.cache.behind.reads}} can improve performance
> substantially on many MapReduce jobs. In our internal benchmarks, we have
> seen speedups of 40% on certain workloads. The reason is because if we know
> the block data will not be read again any time soon, keeping it out of memory
> allows more memory to be used by the other processes on the system. See
> HADOOP-7714 for more benchmarks.
> We would like to turn on these configurations on a per-file or per-client
> basis, rather than on the DataNode as a whole. This will allow more users to
> actually make use of them. It would also be good to add unit tests for the
> drop-cache code path, to ensure that it is functioning as we expect.
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