B.4. Region Overlap Repairs

Table integrity problems can require repairs that deal with overlaps. This is a riskier operation because it requires modifications to the file system, requires some decision making, and may require some manual steps. For these repairs it is best to analyze the output of a hbck -details run so that you isolate repairs attempts only upon problems the checks identify. Because this is riskier, there are safeguard that should be used to limit the scope of the repairs. WARNING: This is a relatively new and have only been tested on online but idle HBase instances (no reads/writes). Use at your own risk in an active production environment! The options for repairing table integrity violations include:

When repairing overlapping regions, a region’s data can be modified on the file system in two ways: 1) by merging regions into a larger region or 2) by sidelining regions by moving data to “sideline” directory where data could be restored later. Merging a large number of regions is technically correct but could result in an extremely large region that requires series of costly compactions and splitting operations. In these cases, it is probably better to sideline the regions that overlap with the most other regions (likely the largest ranges) so that merges can happen on a more reasonable scale. Since these sidelined regions are already laid out in HBase’s native directory and HFile format, they can be restored by using HBase’s bulk load mechanism. The default safeguard thresholds are conservative. These options let you override the default thresholds and to enable the large region sidelining feature.

Since often times you would just want to get the tables repaired, you can use this option to turn on all repair options:

Finally, there are safeguards to limit repairs to only specific tables. For example the following command would only attempt to check and repair table TableFoo and TableBar.

$ ./bin/hbase hbck -repair TableFoo TableBar

B.4.1. Special cases: Meta is not properly assigned

There are a few special cases that hbck can handle as well. Sometimes the meta table’s only region is inconsistently assigned or deployed. In this case there is a special -fixMetaOnly option that can try to fix meta assignments.

$ ./bin/hbase hbck -fixMetaOnly -fixAssignments

B.4.2. Special cases: HBase version file is missing

HBase’s data on the file system requires a version file in order to start. If this flie is missing, you can use the -fixVersionFile option to fabricating a new HBase version file. This assumes that the version of hbck you are running is the appropriate version for the HBase cluster.

B.4.3. Special case: Root and META are corrupt.

The most drastic corruption scenario is the case where the ROOT or META is corrupted and HBase will not start. In this case you can use the OfflineMetaRepair tool create new ROOT and META regions and tables. This tool assumes that HBase is offline. It then marches through the existing HBase home directory, loads as much information from region metadata files (.regioninfo files) as possible from the file system. If the region metadata has proper table integrity, it sidelines the original root and meta table directories, and builds new ones with pointers to the region directories and their data.

$ ./bin/hbase org.apache.hadoop.hbase.util.hbck.OfflineMetaRepair

NOTE: This tool is not as clever as uberhbck but can be used to bootstrap repairs that uberhbck can complete. If the tool succeeds you should be able to start hbase and run online repairs if necessary.

B.4.4. Special cases: Offline split parent

Once a region is split, the offline parent will be cleaned up automatically. Sometimes, daughter regions are split again before their parents are cleaned up. HBase can clean up parents in the right order. However, there could be some lingering offline split parents sometimes. They are in META, in HDFS, and not deployed. But HBase can't clean them up. In this case, you can use the -fixSplitParents option to reset them in META to be online and not split. Therefore, hbck can merge them with other regions if fixing overlapping regions option is used.

This option should not normally be used, and it is not in -fixAll.

comments powered by Disqus