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https://issues.apache.org/jira/browse/HADOOP-894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12494608
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Konstantin Shvachko commented on HADOOP-894:
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Yes. LocatedBlocks contains file length and a List of block locations.
I initially implemented open(src, length) because it is more general, and
deprecated old open(src).
Dhruba finds it "not very intuitive" and Sameer says it does not "add a lot of
value".
I cannot implement open(start,length) with the start > 0 right now, because in
order to do that I will
need to write an offset-to-block map for cached blocks in the client. I was
planning to do it in the next
iteration, but it was supposed to be used mostly in pread() that is for
getBlockLocations(), not in open().
I don't see how we can benefit from introducing the start parameter, but I
definitely support adding length.
So currently it's a tie 2:2. We need more votes to resolve the issue.
> 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
> 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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