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From "Chuan Liu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-8564) Create a Windows native InputStream class to address datanode concurrent reading and writing issue
Date Thu, 05 Jul 2012 18:50:34 GMT
Chuan Liu created HADOOP-8564:
---------------------------------

             Summary: 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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