[ http://issues.apache.org/jira/browse/HADOOP-519?page=all ]
Doug Cutting updated HADOOP-519:
--------------------------------
Status: Open (was: Patch Available)
Overall this looks like a great addition that is well-implemented. A few nits:
In DFSClient, you've duplicated a lot of code from blockSeekTo in
fetchBlockByteRange. Can you perhaps instead add one or two more methods that
capture this common code?
The javadoc for read(long,byte[], int,int) should say "read up to" or "attempt
to read", since it may not read all of the bytes (that's what readFully is for).
The javadoc comments on the new FSInputStream methods do not add anything
useful to what would be inherited from the interface, and what they do add
makes them inappropriate for inheritance by subclasses. So these should be
removed.
> 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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