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https://issues.apache.org/jira/browse/HADOOP-894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12494581
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Doug Cutting commented on HADOOP-894:
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Sameer: you're right, our current public API would not take advantage of an
open with start and length, so it may be overkill. And in many cases we also
read a file header from the first block before we seek anyway. Long-term, this
might be a good optimization, to be able to open a file directly at a position,
without touching the first block, and to be able to disable the reading of
headers. It would be convenient if this did not require changes to both the
protocol and to the server, but instead only on the client. To me,
open(start,length) is a more general API that's no harder to implement than
open(length), one that's future compatible. The client would, for now, always
pass zero for 'start'. But I wouldn't veto open(length). That's also a fine
API and is more minimal, a good thing.
> 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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