[GitHub] [hbase-operator-tools] joshelser commented on a change in pull request #46: HBASE-23562 [operator tools] Add a RegionsMerge tool that allows for …

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joshelser commented on a change in pull request #46: HBASE-23562 [operator tools] Add a RegionsMerge tool that allows for …
URL: https://github.com/apache/hbase-operator-tools/pull/46#discussion_r389960883
 
 

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 File path: hbase-tools/README.md
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+
+# Apache HBase Tool for merging regions
+
+_RegionsMerger_ is an utility tool for manually merging bunch of regions of
+a given table. It's mainly useful on situations when an HBase cluster has too
+many regions per RegionServers, and many of these regions are small enough that
+it can be merged together, reducing the total number of regions in the cluster
+and releasing RegionServers overall memory resources.
+
+This may happen for mistakenly pre-splits, or after a purge in table
+data, as regions would not be automatically merged.
+
+## Setup
+Make sure HBase tools jar is added to HBase classpath:
+
+```
+export HBASE_CLASSPATH=$HBASE_CLASSPATH:./hbase-tools-1.1.0-SNAPSHOT.jar
+```
+
+## Usage
+
+_RegionsMerger_ requires two arguments as parameters: 1) The name of the table
+to have regions merged; 2) The desired total number of regions for the informed
+table. For example, to merge all regions of table `my-table` until it gets to a
+total of 5 regions, assuming the _setup_ step above has been performed:
+
+```
+$ hbase org.apache.hbase.RegionsMerger my-table 5
+```
+
+## Implementation Details
+
+_RegionsMerger_ uses client API
+_org.apache.hadoop.hbase.client.Admin.getRegions_ to fetch the list of regions
+for the specified table, iterates through the resulting list, identifying pairs
+of adjacent regions. For each pair found, it submits a merge request using
+_org.apache.hadoop.hbase.client.Admin.mergeRegionsAsync_ client API method.
+This means multiple merge requests had been sent once the whole list has been
+iterated.
+
+Assuming all these requests complete, resulting total number of
+regions will be ceil((original total number regions)/2). This resulting total
 
 Review comment:
   nit: use english instead of pseudocode for a README.
   
   > Assuming that all merges issued by the RegionsMerger are successful, the resulting number of regions will be no more than half the original number of regions.

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