hbase DroppedSnapshotException 异常

同步超时，很明确的错误。是不是同步调用中有耗时的地方。

DroppedSnapshotException这个异常一般是flushRegion中出现的异常，关于这个异常可以查看jira中这个异常的修复HBASE-644
此外，出现这种异常，查看memstore是否超过阈值，以及GC等

private boolean flushRegion(final Region region, final boolean emergencyFlush,

  boolean forceFlushAllStores) {
long startTime = 0;
synchronized (this.regionsInQueue) {
  FlushRegionEntry fqe = this.regionsInQueue.remove(region);
  // Use the start time of the FlushRegionEntry if available
  if (fqe != null) {
    startTime = fqe.createTime;
  }
  if (fqe != null && emergencyFlush) {
    // Need to remove from region from delay queue.  When NOT an
    // emergencyFlush, then item was removed via a flushQueue.poll.
    flushQueue.remove(fqe);
 }
}
if (startTime == 0) {
  // Avoid getting the system time unless we don't have a FlushRegionEntry;
  // shame we can't capture the time also spent in the above synchronized
  // block
  startTime = EnvironmentEdgeManager.currentTime();
}
lock.readLock().lock();
try {
  notifyFlushRequest(region, emergencyFlush);
  FlushResult flushResult = region.flush(forceFlushAllStores);
  boolean shouldCompact = flushResult.isCompactionNeeded();
  // We just want to check the size
  boolean shouldSplit = ((HRegion)region).checkSplit() != null;
  if (shouldSplit) {
    this.server.compactSplitThread.requestSplit(region);
  } else if (shouldCompact) {
    server.compactSplitThread.requestSystemCompaction(
        region, Thread.currentThread().getName());
  }
  if (flushResult.isFlushSucceeded()) {
    long endTime = EnvironmentEdgeManager.currentTime();
    server.metricsRegionServer.updateFlushTime(endTime - startTime);
  }
} catch (DroppedSnapshotException ex) {
  // Cache flush can fail in a few places. If it fails in a critical
  // section, we get a DroppedSnapshotException and a replay of wal
  // is required. Currently the only way to do this is a restart of
  // the server. Abort because hdfs is probably bad (HBASE-644 is a case
  // where hdfs was bad but passed the hdfs check).
  server.abort("Replay of WAL required. Forcing server shutdown", ex);
  return false;
} catch (IOException ex) {
  LOG.error("Cache flush failed" + (region != null ? (" for region " +
      Bytes.toStringBinary(region.getRegionInfo().getRegionName())) : ""),
    RemoteExceptionHandler.checkIOException(ex));
  if (!server.checkFileSystem()) {
    return false;
  }
} finally {
  lock.readLock().unlock();
  wakeUpIfBlocking();
}
return true;

}

此处官方对于DroppedSnapshotException给出的建议是重启服务，这个有可能是HDFS异常，而在健康检查的时候没有检测出来；以上是个人看法