HDFS中两种random read比较.pdf

HDFS 中两种random read 比较 HDFS 中两种random read 比较 code version: hadoop-0.19.1 首先说pread。pread 会明确的把要读的size 传给datanode （在new BlockReader 的时候） Java 代码 /** * Read bytes starting from the specified position. * * @param position start read from this position * @param buffer read buffer * @param offset offset into buffer * @param length number of bytes to read * * @return actual number of bytes read */ @Override public int read(long position, byte[] buffer, int offset, int length) throws IOException { // sanity checks checkOpen(); if (closed) { throw new IOException(Stream closed); } long filelen = getFileLength(); if ((position 0) || (position = filelen)) { return -1; } int realLen = length; if ((position + length) filelen) { realLen = (int)(filelen - position); } // determine the block and byte range within the block // corresponding to position and realLen ListLocatedBlock blockRange = getBlockRange(position, realLen); int remaining = realLen; for (LocatedBlock blk : blockRange) { long targetStart = position - blk.getStartOffset(); long bytesToRead = Math.min(remaining, blk.getBlockSize() -