HBase hbck——检察HBase集群的一致性
HBase提供了hbck命令来检查各种不一致问题。hbck的名字仿效了HDFS的fsck命令，后者是一个用于检查HDFS中不一致问题的工具。下面这段非常易懂的介绍出自于hbck的源程序。
检查数据在Master及RegionServer的内存中状态与数据在HDFS中的状态之间的一致性。
HBase的hbck不仅能够检查不一致问题，而且还能够修复不一致问题。
在生产环境中，应当经常运行hbck，以便及早发现不一致问题并更容易地解决问题。
一、问题
首先，在HBase上创建一张表usertable。
然后，在HDFS上直接删除usertable表的目录/hbase/usertable来删除表的数据（错误的做法）。
二、解决方法
1.hbase hbck
Status：OK，表示没有发现不一致问题。
Status：INCONSISTENT，表示有不一致问题。
2.hbase hbck -fixAssignments
Try to fix region assignments. Replaces the old -fix
[zk: 192.168.147.15:2181(CONNECTED) 3] ls /hbase/unassigned
3.hbase hbck -fixMeta
Try to fix meta problems. This assumes HDFS region info is good.
hbase(main):007:0> scan '.META.'
4.hbase(main):015:0> create 'usertable','f1'
ERROR: Table already exists: usertable!
[zk: 192.168.147.15:2181(CONNECTED) 4] ls /hbase/table
[usertable]
[zk: 192.168.147.15:2181(CONNECTED) 5] get /hbase/table/usertable
ENABLING
[zk: 192.168.147.15:2181(CONNECTED) 6] rmr /hbase/table/usertable
[zk: 192.168.147.15:2181(CONNECTED) 7] get /hbase/table/usertable
Node does not exist: /hbase/table/usertable
此时，create 'usertable','f1'，依然提示同样的错误。
查看ZKTable源码：
/**
* Cache of what we found in zookeeper so we don't have to go to zk ensemble
* for every query. Synchronize access rather than use concurrent Map because
* synchronization needs to span query of zk.
*/
private final Map<String, TableState> cache =
new HashMap<String, TableState>();
this.cache.put(tableName, state);
把ZK中的数据缓存在本地内存中，以加速访问，减少访问时间。
为了让本地内存中的数据失效，必须重启HBase集群。
重启HBase集群。
此时，create 'usertable','f1'，没有提示错误，表创建成功。
三、hbck
[bigdata@tbe192168147015 ~]$ hbase hbck -h
Usage: fsck [opts] {only tables}
where [opts] are:
-help Display help options (this)
-details Display full report of all regions.
-timelag {timeInSeconds} Process only regions that have not experienced any metadata updates in the last {{timeInSeconds} seconds.
-sleepBeforeRerun {timeInSeconds} Sleep this many seconds before checking if the fix worked if run with -fix
-summary Print only summary of the tables and status.
-metaonly Only check the state of ROOT and META tables.
Repair options: (expert features, use with caution!)
-fix Try to fix region assignments. This is for backwards compatiblity
-fixAssignments Try to fix region assignments. Replaces the old -fix
-fixMeta Try to fix meta problems. This assumes HDFS region info is good.
-fixHdfsHoles Try to fix region holes in hdfs.
-fixHdfsOrphans Try to fix region dirs with no .regioninfo file in hdfs
-fixHdfsOverlaps Try to fix region overlaps in hdfs.
-fixVersionFile Try to fix missing hbase.version file in hdfs.
-maxMerge <n> When fixing region overlaps, allow at most <n> regions to merge. (n=5 by default)
-sidelineBigOverlaps When fixing region overlaps, allow to sideline big overlaps
-maxOverlapsToSideline <n> When fixing region overlaps, allow at most <n> regions to sideline per group. (n=2 by default)
-repair Shortcut for -fixAssignments -fixMeta -fixHdfsHoles -fixHdfsOrphans -fixHdfsOverlaps -fixVersionFile -sidelineBigOverlaps
-repairHoles Shortcut for -fixAssignments -fixMeta -fixHdfsHoles -fixHdfsOrphans