ReadaheadPool (Hadoop 0.20.2-cdh3u3 API)
Date: 2012-03-20

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## org.apache.hadoop.io
Class ReadaheadPool

    java.lang.Object
      org.apache.hadoop.io.ReadaheadPool

    public class ReadaheadPool

extends Object

Manages a pool of threads which can issue readahead requests on file descriptors.

Nested Class Summary
`static interface` | `ReadaheadPool.ReadaheadRequest` An outstanding readahead request that has been submitted to the pool.

Method Summary
`static ReadaheadPool` | `getInstance()` Return the singleton instance for the current process.
`ReadaheadPool.ReadaheadRequest` | `readaheadStream(String identifier, FileDescriptor fd, long curPos, long readaheadLength, long maxOffsetToRead, ReadaheadPool.ReadaheadRequest lastReadahead)` Issue a request to readahead on the given file descriptor.
`ReadaheadPool.ReadaheadRequest` | `submitReadahead(String identifier, FileDescriptor fd, long off, long len)` Submit a request to readahead on the given file descriptor.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### getInstance

    public static ReadaheadPool getInstance()

  Return the singleton instance for the current process.

### readaheadStream

    public ReadaheadPool.ReadaheadRequest readaheadStream(String identifier,
                                                          FileDescriptor fd,
                                                          long curPos,
                                                          long readaheadLength,
                                                          long maxOffsetToRead,
                                                          ReadaheadPool.ReadaheadRequest lastReadahead)

  Issue a request to readahead on the given file descriptor.

  Parameters:
    `identifier` - a textual identifier that will be used in error messages (e.g. the file name)
    `fd` - the file descriptor to read ahead
    `curPos` - the current offset at which reads are being issued
    `readaheadLength` - the configured length to read ahead
    `maxOffsetToRead` - the maximum offset that will be readahead (useful if, for example, only some segment of the file is requested by the user). Pass `Long.MAX_VALUE` to allow readahead to the end of the file.
    `lastReadahead` - the result returned by the previous invocation of this function on this file descriptor, or null if this is the first call
  Returns:
    an object representing this outstanding request, or null if no readahead was performed

### submitReadahead

    public ReadaheadPool.ReadaheadRequest submitReadahead(String identifier,
                                                          FileDescriptor fd,
                                                          long off,
                                                          long len)

  Submit a request to readahead on the given file descriptor.

  Parameters:
    `identifier` - a textual identifier used in error messages, etc.
    `fd` - the file descriptor to readahead
    `off` - the offset at which to start the readahead
    `len` - the number of bytes to read
  Returns:
    an object representing this pending request

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