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https://issues.apache.org/jira/browse/HADOOP-894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12494094
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Doug Cutting commented on HADOOP-894:
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When we open a file we don't need anything in return except the length, since
we can call getBlockLocations() afterwards. If we want some block locations
returned from open(), as an optimization, then we should pass a start and
length, giving the range of the file whose blocks we'd initially like, and
return those with the length. HADOOP-1298 will add more fields to DFSFileInfo,
things we don't need when opening. So HADOOP-1298 argues that we should not
return a DFSFileInfo at open. Also, other users of DFSFileInfo don't need a
LocatedBlockList, so I really don't think it belongs there.
> dfs client protocol should allow asking for parts of the block map
> ------------------------------------------------------------------
>
> Key: HADOOP-894
> URL: https://issues.apache.org/jira/browse/HADOOP-894
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Reporter: Owen O'Malley
> Assigned To: Konstantin Shvachko
> Fix For: 0.13.0
>
> Attachments: partialBlockList.patch, partialBlockList2.patch
>
>
> I think that the HDFS client protocol should change like:
> /** The meta-data about a file that was opened. */
> class OpenFileInfo {
> /** the info for the first block */
> public LocatedBlockInfo getBlockInfo();
> public long getBlockSize();
> public long getLength();
> }
> interface ClientProtocol extends VersionedProtocol {
> public OpenFileInfo open(String name) throws IOException;
> /** get block info for any range of blocks */
> public LocatedBlockInfo[] getBlockInfo(String name, int blockOffset, int
> blockLength) throws IOException;
> }
> so that the client can decide how much block info to request and when.
> Currently, when the file is opened or an error occurs, the entire block list
> is requested and sent.
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