[jira] Updated: (HADOOP-519) HDFS File API should be extended to include positional read

["Milind Bhandarkar (JIRA)" <ji...@apache.org>]

     [ http://issues.apache.org/jira/browse/HADOOP-519?page=all ]

Milind Bhandarkar updated HADOOP-519:
-------------------------------------

    Attachment: pread.patch

This patch implements positional read for FSInputStream, FSDataInputStream, DFSInputStream, and LocalFSFileInputStream.

I will later re-implement "normal" read in terms of pread in a separate patch.

> 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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