Chuan Liu created HADOOP-8564:
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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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