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https://issues.apache.org/jira/browse/HDFS-347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13630905#comment-13630905
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Aaron T. Myers commented on HDFS-347:
-------------------------------------
Nicholas,
There is quite a bit of precedent for merging a branch to trunk before all of
the sub-tasks are completed, if the branch as-is is functional and won't break
trunk and those sub-tasks can reasonably be continued on trunk, e.g. HDFS-1623,
HDFS-3077, and HDFS-1073. Given that Suresh had suggested in response to your
-1 on the merge vote that we continue working on this feedback on trunk, and
given that you then removed your -1 and the vote passed, it seems to me like
this branch was ready for a merge from trunk.
Yes, there are a few more small cleanup sub-tasks to work on, but that can be
done on trunk. Let's continue the work there.
> DFS read performance suboptimal when client co-located on nodes with data
> -------------------------------------------------------------------------
>
> Key: HDFS-347
> URL: https://issues.apache.org/jira/browse/HDFS-347
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode, hdfs-client, performance
> Reporter: George Porter
> Assignee: Colin Patrick McCabe
> Fix For: 3.0.0
>
> Attachments: 2013.01.28.design.pdf, 2013.01.31.consolidated2.patch,
> 2013.01.31.consolidated.patch, 2013.02.15.consolidated4.patch,
> 2013-04-01-jenkins.patch, all.tsv, a.patch, BlockReaderLocal1.txt,
> full.patch, HADOOP-4801.1.patch, HADOOP-4801.2.patch, HADOOP-4801.3.patch,
> HDFS-347-016_cleaned.patch, HDFS-347.016.patch, HDFS-347.017.clean.patch,
> HDFS-347.017.patch, HDFS-347.018.clean.patch, HDFS-347.018.patch2,
> HDFS-347.019.patch, HDFS-347.020.patch, HDFS-347.021.patch,
> HDFS-347.022.patch, HDFS-347.024.patch, HDFS-347.025.patch,
> HDFS-347.026.patch, HDFS-347.027.patch, HDFS-347.029.patch,
> HDFS-347.030.patch, HDFS-347.033.patch, HDFS-347.035.patch,
> HDFS-347-branch-20-append.txt, hdfs-347-merge.txt, hdfs-347-merge.txt,
> hdfs-347-merge.txt, hdfs-347.png, hdfs-347.txt, local-reads-doc
>
>
> One of the major strategies Hadoop uses to get scalable data processing is to
> move the code to the data. However, putting the DFS client on the same
> physical node as the data blocks it acts on doesn't improve read performance
> as much as expected.
> After looking at Hadoop and O/S traces (via HADOOP-4049), I think the problem
> is due to the HDFS streaming protocol causing many more read I/O operations
> (iops) than necessary. Consider the case of a DFSClient fetching a 64 MB
> disk block from the DataNode process (running in a separate JVM) running on
> the same machine. The DataNode will satisfy the single disk block request by
> sending data back to the HDFS client in 64-KB chunks. In BlockSender.java,
> this is done in the sendChunk() method, relying on Java's transferTo()
> method. Depending on the host O/S and JVM implementation, transferTo() is
> implemented as either a sendfilev() syscall or a pair of mmap() and write().
> In either case, each chunk is read from the disk by issuing a separate I/O
> operation for each chunk. The result is that the single request for a 64-MB
> block ends up hitting the disk as over a thousand smaller requests for 64-KB
> each.
> Since the DFSClient runs in a different JVM and process than the DataNode,
> shuttling data from the disk to the DFSClient also results in context
> switches each time network packets get sent (in this case, the 64-kb chunk
> turns into a large number of 1500 byte packet send operations). Thus we see
> a large number of context switches for each block send operation.
> I'd like to get some feedback on the best way to address this, but I think
> providing a mechanism for a DFSClient to directly open data blocks that
> happen to be on the same machine. It could do this by examining the set of
> LocatedBlocks returned by the NameNode, marking those that should be resident
> on the local host. Since the DataNode and DFSClient (probably) share the
> same hadoop configuration, the DFSClient should be able to find the files
> holding the block data, and it could directly open them and send data back to
> the client. This would avoid the context switches imposed by the network
> layer, and would allow for much larger read buffers than 64KB, which should
> reduce the number of iops imposed by each read block operation.
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