hadoop之DataBlockScanner

每一个DataNode都会初始化一个数据块扫描器(DataBlockScanner)，用于周期性检测DataNode上的存储的所有数据块的正确性。如果发现有损坏的数据块则报告给NameNode。由于DataNode的数据块都是放在块池里面的，所以会持有一个BlockPoolSliceScanner对象，每一个BlockPoolSliceScanner对象负责验证一个指定的块池下数据块的正确性和完整性.

 

2.5版本的数据块扫描,其实2.6版本核心的扫描方法是一样的，都是通过发送一个数据到空的输出流上

DataBlockScanner:

private final TreeMap<String, BlockPoolSliceScanner> blockPoolScannerMap =  new TreeMap<String, BlockPoolSliceScanner>();public void run() {    String currentBpId = "";    boolean firstRun = true;    //除第一次遍历外，以后每次要sleep 5s    while (datanode.shouldRun && !Thread.interrupted()) {        if (!firstRun) {            try {                Thread.sleep(SLEEP_PERIOD_MS);            } catch (InterruptedException ex) {                // Interrupt itself again to set the interrupt status                blockScannerThread.interrupt();                continue;            }        } else {            firstRun = false;        }        //选择一个距离现在时间最长的BlockPoolSliceScanner        BlockPoolSliceScanner bpScanner = getNextBPScanner(currentBpId);        //......省略        //调用BlockPoolSliceScanner的scanBlockPoolSlice进行扫描        bpScanner.scanBlockPoolSlice();    }    //......省略}private BlockPoolSliceScanner getNextBPScanner(String currentBpId) {    String nextBpId = null;    while (datanode.shouldRun && !blockScannerThread.isInterrupted()) {        waitForInit();        synchronized (this) {            if (getBlockPoolSetSize() > 0) {                //寻找一个距离现在时间最长的BlockPoolSliceScanner                long lastScanTime = 0;                for (String bpid : blockPoolScannerMap.keySet()) {                    final long t = getBPScanner(bpid).getLastScanTime();                    if (t != 0L) {                        if (bpid == null || t < lastScanTime) {                            lastScanTime =  t;                            nextBpId = bpid;                        }                    }                }                //......省略            }        }        //......省略    }    return null;}

 

BlockPoolSliceScanner:

voidscanBlockPoolSlice() {
    //......省略
    // Start scanning
    scan();
}
private void scan() {
    //调整节流器
    adjustThrottler();

    while (datanode.shouldRun
      && !datanode.blockScanner.blockScannerThread.isInterrupted()
      && datanode.isBPServiceAlive(blockPoolId)){
        long now = Time.monotonicNow();
        synchronized(this) {
            if (now >= (currentPeriodStart+ scanPeriod)) {
                startNewPeriod();
            }
        }
        if (((now - getEarliestScanTime())>= scanPeriod)
          || ((!blockInfoSet.isEmpty())&& !(this.isFirstBlockProcessed()))) {
            //挑一个block验证
            verifyFirstBlock();
        } else {
            //......省略
        }
    }
}

private voidverifyFirstBlock() {
    Block block = null;
    synchronized (this) {
        if (!blockInfoSet.isEmpty()){
            block = blockInfoSet.first();
        }
    }
    if ( block != null ) {
        //校验block
        verifyBlock(new ExtendedBlock(blockPoolId,block));
        processedBlocks.put(block.getBlockId(),1);
    }
}

void verifyBlock(ExtendedBlock block) {
    BlockSender blockSender = null;
    for (int i=0; i<2; i++){
        boolean second = (i> 0);
        try {
            //调整节流器
            adjustThrottler();
            //创建BlockSender，将数据块发送到空的输出流中
            blockSender = new BlockSender(block,0,-1, false, true, true,
                datanode, null, CachingStrategy.newDropBehind());
            DataOutputStream out =
              new DataOutputStream(newIOUtils.NullOutputStream());
            blockSender.sendBlock(out,null,throttler);
        if ( second ) {
            totalTransientErrors++;
        }
        //数据块校验正确，更新数据块扫描状态
        updateScanStatus(block.getLocalBlock(),ScanType.VERIFICATION_SCAN, true);
        return;
        } catch (IOException e){
            updateScanStatus(block.getLocalBlock(),ScanType.VERIFICATION_SCAN, false);

            //如果FsDataSet数据块不存在，则删除这个快
            if (!dataset.contains(block)) {
                LOG.info(block+ " isno longer in the dataset");
                deleteBlock(block.getLocalBlock());
                return;
            }

            if (e instanceofFileNotFoundException ) {
                LOG.info("Verification failed for "+ block +
                  " - may be due to race with write");
                deleteBlock(block.getLocalBlock());
                return;
            }
            //如果2次检查数据块的checksum都出错了，调用handleScanFailure通知NameNode这个数据块已经损坏
            if (second) {
                totalScanErrors++;
                datanode.getMetrics().incrBlockVerificationFailures();
                handleScanFailure(block);
                return;
            }
        } finally {
            IOUtils.closeStream(blockSender);
            datanode.getMetrics().incrBlocksVerified();
            totalScans++;
        }
    }
}