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https://issues.apache.org/jira/browse/HADOOP-8564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13413867#comment-13413867
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Todd Lipcon commented on HADOOP-8564:
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Hi Chuan. Thanks for taking a look into that.
I wouldn't be concerned about API compatibility here, since these are
private-facing (internal) APIs. We can change them between versions without
breaking any contracts with downstream projects.
I think your idea of separating the windows calls from the POSIX ones makes
sense. But, we should probably also enumerate the uses of the POSIX calls and
figure out what the equivalents are on the Windows side - for example, we use
fstat and open(O_EXCL) for a lot of security reasons. I don't know for sure
whether Windows has equivalent APIs or we need to take another route entirely
in those situations.
> Create a Windows native InputStream class to address datanode concurrent
> reading and writing issue
> --------------------------------------------------------------------------------------------------
>
> Key: HADOOP-8564
> URL: https://issues.apache.org/jira/browse/HADOOP-8564
> Project: Hadoop Common
> Issue Type: Bug
> Components: io
> Affects Versions: 1-win
> Reporter: Chuan Liu
> Assignee: Chuan Liu
>
> HDFS files are made up of blocks. First, let’s look at writing. When the data
> is written to datanode, an active or temporary file is created to receive
> packets. After the last packet for the block is received, we will finalize
> the block. One step during finalization is to rename the block file to a new
> directory. The relevant code can be found via the call sequence:
> FSDataSet.finalizeBlockInternal -> FSDir.addBlock.
> {code}
> if ( ! metaData.renameTo( newmeta ) ||
> ! src.renameTo( dest ) ) {
> throw new IOException( "could not move files for " + b +
> " from tmp to " +
> dest.getAbsolutePath() );
> }
> {code}
> Let’s then switch to reading. On HDFS, it is expected the client can also
> read these unfinished blocks. So when the read calls from client reach
> datanode, the datanode will open an input stream on the unfinished block file.
> The problem comes in when the file is opened for reading while the datanode
> receives last packet from client and try to rename the finished block file.
> This operation will succeed on Linux, but not on Windows . The behavior can
> be modified on Windows to open the file with FILE_SHARE_DELETE flag on, i.e.
> sharing the delete (including renaming) permission with other processes while
> opening the file. There is also a Java bug ([id
> 6357433|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6357433]) reported
> a while back on this. However, since this behavior exists for Java on Windows
> since JDK 1.0, the Java developers do not want to break the backward
> compatibility on this behavior. Instead, a new file system API is proposed in
> JDK 7.
> As outlined in the [Java forum|http://www.java.net/node/645421] by the Java
> developer (kbr), there are three ways to fix the problem:
> # Use different mechanism in the application in dealing with files.
> # Create a new implementation of InputStream abstract class using Windows
> native code.
> # Patch JDK with a private patch that alters FileInputStream behavior.
> For the third option, it cannot fix the problem for users using Oracle JDK.
> We discussed some options for the first approach. For example one option is
> to use two phase renaming, i.e. first hardlink; then remove the old hardlink
> when read is finished. This option was thought to be rather pervasive.
> Another option discussed is to change the HDFS behavior on Windows by not
> allowing client reading unfinished blocks. However this behavior change is
> thought to be problematic and may affect other application build on top of
> HDFS.
> For all the reasons discussed above, we will use the second approach to
> address the problem.
> If there are better options to fix the problem, we would also like to hear
> about them.
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