速记：
1. 现象：
线下有一个5个Region节点的Hbase集群（小内存），昨日全挂掉了。重启，启动不了。
2. 原因
查看错误日志，发现出发点是因为GC超时导致。
首先是其中某一个节点发生GC，连接ZK超时，连接关闭而挂掉。
挂掉后发生了Region迁移，如下：
Adding moved region record: 41810a8da6ae1fa59a732d24a521d55c to yq-hadoop***,60020,1520389720296 as of 405597
Adding moved region record: 9cfe6192b4c7074699177ff2a5b72e70 to yq-hadoop***,60020,1520389720296 as of 1605
Region 迁移之后，其他节点负载增加，GC更加严重，相继挂掉。
我们知道，hbase 是比较耗内存的，小内存是其软肋。其实除了并发有点高之外，主要原因还是因为测试集群机身内存太小了（只有7GB），加之其他应用，HBASE 堆栈无法分配过多内存，导致GC严重。
3. 重启失败
There is insufficient memory for the Java Runtime Environment to continue.
Native memory allocation (malloc) failed to allocate 715784192 bytes for committing reserved memory.
出现上述常见原因有三：
1. 确实是机身内存不足以分配相应内存
2. 某进程启动线程过多，可能是代码哪里有问题
3. ulimit -n 过小
本文正是因为第一个原因。
解决办法：
停掉此节点无用的应用，释放更多内存。
调小Hbase Region Server 堆栈内存，如下：
image.png
- 重启即可。
4. 预警信息
当发现日志中出现如下提示信息（flush 延迟），一般是内存分配不足了，要早做处理。
2018-03-07 10:20:38,091 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,6\x00\x00\x00\x00\x00\x00\x00,1499652593538.3e9884dcb01d4a98408919fbc3433f3f. because S has an old edit so flush to free WALs after random delay 33552ms
2018-03-07 10:20:38,091 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,\xB4\x00\x00\x00\x00\x00\x00\x00,1499652593538.f7538a5b9b88fa60857a9f38f272a436. because S has an old edit so flush to free WALs after random delay 242347ms
2018-03-07 10:20:38,091 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,{\x00\x00\x00\x00\x00\x00\x00,1499652593538.fb9d550b69cc3409d5256718c47d871d. because S has an old edit so flush to free WALs after random delay 118059ms
2018-03-07 10:20:38,091 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,;\x00\x00\x00\x00\x00\x00\x00,1499652593538.3c226c0465be2a64d7c0fa9e25e41a7e. because S has an old edit so flush to free WALs after random delay 135722ms
2018-03-07 10:20:38,092 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,\xA8\x00\x00\x00\x00\x00\x00\x00,1499652593538.90c4ebec39bb67680dbe4f17c662d229. because S has an old edit so flush to free WALs after random delay 287717ms
2018-03-07 10:20:38,092 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,<\x00\x00\x00\x00\x00\x00\x00,1499652593538.2bd2b43e949edd374878fe6b31f79c15. because S has an old edit so flush to free WALs after random delay 280345ms
2018-03-07 10:20:38,092 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,\x88\x00\x00\x00\x00\x00\x00\x00,1499652593538.e00b1e90a58c0f8d78fe4727d87c8949. because S has an old edit so flush to free WALs after random delay 150252ms
2018-03-07 10:20:38,092 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: yq-hadoop****,60020,1520385422866-MemstoreFlusherChore requesting flush of TraceV2,\xF2\x00\x00\x00\x00\x00\x00\x00,1499652593538.d834224abc9d6b88b93911ea1fa17a0e. because S has an old edit so flush to free WALs after random delay 230071ms
