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https://issues.apache.org/jira/browse/HADOOP-894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12493508
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Doug Cutting commented on HADOOP-894:
-------------------------------------
I think it's strange to put LocatedBlocks in DFSFileInfo. You're trying to
optimize the protocol, so that a separate call isn't required to get the
length, right? So let's make that explicit by returning the file length along
with the list of blocks, rather than hacking DFSFileInfo.
public LocatedBlocks {
private LocatedBlock[] blocks;
private long fileLength;
}
public LocatedBlocks getBlockLocations(String file, long start, long length);
Then we don't need the open() method at all. getBlockLocations() replaces it
altogether. This also has the benefit that someone can open a file in the
middle with a single RPC.
> 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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