hbase夯机问题：
FATAL org.apache.hadoop.hbase.regionserver.HRegionServer: ABORTING region server : Replay of WAL required. Forcing server shutdown?
Caused by: org.apache.hadoop.hbase.exceptions.TimeoutIOException: Failed to get sync result after ms for ringBufferSequence=, WAL system stuck?
解决思路：
一：Memstore Flush触发条件
HBase会在如下几种情况下触发flush操作，需要注意的是MemStore的最小flush单元是HRegion而不是单个MemStore。可想而知，如果一个HRegion中Memstore过多，每次flush的开销必然会很大，因此我们也建议在进行表设计的时候尽量减少ColumnFamily的个数。
Memstore级别限制：当Region中任意一个MemStore的大小达到了上限（hbase.hregion.memstore.flush.size，默认128MB），会触发Memstore刷新。
Region级别限制：当Region中所有Memstore的大小总和达到了上限（hbase.hregion.memstore.block.multiplier * hbase.hregion.memstore.flush.size，默认 2* 128M = 256M），会触发memstore刷新。
Region Server级别限制：当一个Region Server中所有Memstore的大小总和达到了上限（hbase.regionserver.global.memstore.upperLimit ＊ hbase_heapsize，默认 40%的JVM内存使用量），会触发部分Memstore刷新。Flush顺序是按照Memstore由大到小执行，先Flush Memstore最大的Region，再执行次大的，直至总体Memstore内存使用量低于阈值（hbase.regionserver.global.memstore.lowerLimit ＊ hbase_heapsize，默认 38%的JVM内存使用量）。
当一个Region Server中HLog数量达到上限（可通过参数hbase.regionserver.maxlogs配置）时，系统会选取最早的一个 HLog对应的一个或多个Region进行flush
HBase定期刷新Memstore：默认周期为1小时，确保Memstore不会长时间没有持久化。为避免所有的MemStore在同一时间都进行flush导致的问题，定期的flush操作有20000左右的随机延时。
手动执行flush：用户可以通过shell命令 flush ‘tablename’或者flush ‘region name’分别对一个表或者一个Region进行flush。
二：hbase源码;
> /**
> * Flush a region.
> * @param region Region to flush.
> * @param emergencyFlush Set if we are being force flushed. If true the region
> * needs to be removed from the flush queue. If false, when we were called
> * from the main flusher run loop and we got the entry to flush by calling
> * poll on the flush queue (which removed it).
> * @param forceFlushAllStores whether we want to flush all store.
> * @return true if the region was successfully flushed, false otherwise. If
> * false, there will be accompanying log messages explaining why the region
was
> * not flushed.
> */
> private boolean flushRegion(HRegion region, boolean emergencyFlush, boolean forceFlushAllStores,
> FlushLifeCycleTracker tracker) {
> synchronized (this.regionsInQueue) {
> FlushRegionEntry fqe = this.regionsInQueue.remove(region);
> // Use the start time of the FlushRegionEntry if available
> 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);
> }
> }
> tracker.beforeExecution();
> lock.readLock().lock();
> try {
> notifyFlushRequest(region, emergencyFlush);
> FlushResult flushResult = region.flushcache(forceFlushAllStores, false, tracker);
> boolean shouldCompact = flushResult.isCompactionNeeded();
> // We just want to check the size
> boolean shouldSplit = region.checkSplit() != null;
> if (shouldSplit) {
> this.server.compactSplitThread.requestSplit(region);
> } else if (shouldCompact) {
> server.compactSplitThread.requestSystemCompaction(region, Thread.currentThread().getName());
> }
> } 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) {
> ex = ex instanceof RemoteException ? ((RemoteException) ex).unwrapRemoteException() : ex;
> LOG.error(
> "Cache flush failed"
> + (region != null ? (" for region " +
> Bytes.toStringBinary(region.getRegionInfo().getRegionName()))
> : ""), ex);
> if (!server.checkFileSystem()) {
> return false;
> }
> } finally {
> lock.readLock().unlock();
> wakeUpIfBlocking();
> tracker.afterExecution();
> }
> return true;
> }
>
三：其中失败代码：
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;
}
四：解决方案：
1.设置memstore大小；HloG数量设置；
2.check hdfs 并且修复
3.重启server。
