Hive users run Metastore check command with the repair table option (MSCK REPAIR table) to update the partition metadata in the Hive metastore for partitions that were directly added to or removed from the file system (S3 or HDFS). When run, MSCK repair command must make a file system call to check if the partition exists for each partition. This step could take a long time if the table has thousands of partitions. In EMR 6.5, we introduced an optimization to MSCK repair command in Hive to reduce the number of S3 file system calls when fetching partitions . This feature improves performance of MSCK command (~15-20x on 10k+ partitions) due to reduced number of file system calls especially when working on tables with large number of partitions. Previously, you had to enable this feature by explicitly setting a flag. Starting with Amazon EMR 6.8, we further reduced the number of S3 filesystem calls to make MSCK repair run faster and enabled this feature by default.
Source:: Amazon AWS