Apache Hive MSCK

最近在使用Hive的过程中，在备份数据时，经常会使用cp或mv命令来拷贝数据，
将数据拷贝到我们新建备份表的目录下面，如果不是分区表，则上面的操作之后，
新建的备份表可以正常使用，但是如果是分区表的，一般都是使用alter table add partition命令将分区信息添加到新建的表中，每添加一条分区信息就执行一个alter table add partition命令，如果分区数量少还好办，但是遇到分区数量多的情况，特别是分区数量大于50的情况，如果还是使用alter命令添加分区，那是一件耗时耗力的事情，还容易出错。

Hive stores a list of partitions for each table in its metastore. If, however, new partitions are directly added to HDFS (say by using hadoop fs -put command), the metastore (and hence Hive) will not be aware of these partitions unless the user runs ALTER TABLE table_name ADD PARTITION commands on each of the newly added partitions.
However, users can run a metastore check command with the repair table option:
MSCK REPAIR TABLE table_name;
which will add metadata about partitions to the Hive metastore for partitions for which such metadata doesn't already exist. In other words, it will add any partitions that exist on HDFS but not in metastore to the metastore. See HIVE-874 for more details. When there is a large number of untracked partitions, there is a provision to run MSCK REPAIR TABLE batch wise to avoid OOME. By giving the configured batch size for the property hive.msck.repair.batch.size it can run in the batches internally. The default value of the property is zero, it means it will execute all the partitions at once.
The equivalent command on Amazon Elastic MapReduce (EMR)'s version of Hive is:
ALTER TABLE table_name RECOVER PARTITIONS;
Starting with Hive 1.3, MSCK will throw exceptions if directories with disallowed characters in partition values are found on HDFS. Use hive.msck.path.validation setting on the client to alter this behavior; "skip" will simply skip the directories. "ignore" will try to create partitions anyway (old behavior). This may or may not work.

下面介绍如果使用MSCK命令，创建了一个a2data.msck_table表，然后通过cp命令将几个目录的文件拷贝到a2data.msck_table目录下面，然后执行show partitions a2data.msck_table命令，将不会显示分区的信息：

hive> show partitions a2data.msck_table;
OK
Time taken: 1.121 seconds
然后执行MSCK REPAIR TABLE a2data.msck_table;命令添加分区：

hive> MSCK REPAIR TABLE a2data.msck_table;
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 10124
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 21234
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 346783
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 2532162
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 2901198
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 3129087
19/05/25 17:27:15 WARN log: Updating partition stats fast for: a2data.msck_table;
19/05/25 17:27:15 WARN log: Updated size to 23190876
OK
Partitions not in metastore:    a2data.msck_table;:dt=2019-05-15    a2data.msck_table;:dt=2019-05-16    a2data.msck_table;:dt=2019-05-17    a2data.msck_table;:dt=2019-05-18    a2data.msck_table;:dt=2019-05-19     a2data.msck_table;:dt=2019-05-20    a2data.msck_table;:dt=2019-05-21
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-15
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-16
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-17
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-18
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-19
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-20
Repair: Added partition to metastore a2data.msck_table;:dt=2019-05-21
Time taken: 0.613 seconds, Fetched 8 row(s) 
通过上述的结果可以看到已经将a2data.msck_table表的分区信息添加到Hive metastore中，和add partition命令比起来既方便又高效。

有点需要注意的是，分区的目录结构必遵循
/partition_name=partition_value/结构，
否则msck无法自动添加分区，只能使用add partition命令。