[ http://issues.apache.org/jira/browse/HADOOP-519?page=all ]
Milind Bhandarkar updated HADOOP-519:
-------------------------------------
Attachment: pread.patch
Object[] returned pair is replaced by a private static class.
> HDFS File API should be extended to include positional read
> -----------------------------------------------------------
>
> Key: HADOOP-519
> URL: http://issues.apache.org/jira/browse/HADOOP-519
> Project: Hadoop
> Issue Type: New Feature
> Components: dfs
> Affects Versions: 0.6.0
> Environment: All
> Reporter: Milind Bhandarkar
> Assigned To: Milind Bhandarkar
> Fix For: 0.7.0
>
> Attachments: pread.patch
>
>
> HDFS Input streams should support positional read. Positional read (such as
> the pread syscall on linux) allows reading for a specified offset without
> affecting the current file offset. Since the underlying file state is not
> touched, pread can be used efficiently in multi-threaded programs.
> Here is how I plan to implement it.
> Provide PositionedReadable interface, with the following methods:
> int read(long position, byte[] buffer, int offset, int length);
> void readFully(long position, byte[] buffer, int offset, int length);
> void readFully(long position, byte[] buffer);
> Abstract class FSInputStream would provide default implementation of the
> above methods using getPos(), seek() and read() methods. The default
> implementation is inefficient in multi-threaded programs since it locks the
> object while seeking, reading, and restoring to old state.
> DFSClient.DFSInputStream, which extends FSInputStream will provide an
> efficient non-synchronized implementation for above calls.
> In addition, FSDataInputStream, which is a wrapper around FSInputStream, will
> provide wrapper methods for above read methods as well.
> Patch forthcoming early next week.
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