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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-519) HDFS File API should be extended to include positional read
Date Mon, 11 Sep 2006 17:31:24 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-519?page=comments#action_12433910 ] 
Doug Cutting commented on HADOOP-519:

So, to be clear, you will modify both FSInputStream and FSDataInputStream to implement the
new PositionReadable interface, right?  (FSDataInputStream should also implement Seekable
too: it already has the methods, but does not yet reference the interface.)

Only the primitive FSInputStream.read(long,byte[],int,int) method needs to be synchronized.
 The others can be unsynchronized and implemented only in base classes, inherited by optimized

An optimized implementation of read(long,byte[],int,int) can be provided in both DFSInputStream
and LocalFSInputStream (the latter using nio's FileChannel.read(ByteBuffer,long)).  It might
be simpler if the PositionReadble API were instead read(ByteBuffer, long), so that the client
can manage ByteBuffer allocation.

Finally, we should change implementations of read(byte[],int,int) and seek(long) to be synchronized.
 This won't hurt, since they're not currently thread safe, and it will make the positioned-read
methods thread-safe.

> 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
> 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
> 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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