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TiKV replicates a segment of data in Regions via the Raft state machine. As data writes increase, a Region Split happens when the size of the region or the number of keys has reached a threshold. Conversely, if the size of the Region and the amount of keys shrinks because of data deletion, we can use Region Merge to merge adjacent regions that are smaller. This relieves some stress on Raftstore.

Merge process

Region Merge is initiated by the Placement Driver (PD). The steps are:

1. PD polls the meta information of Regions constantly.

2. If the region size is less than max-merge-region-size and the number of keys the region includes is less than max-merge-region-keys, PD performs Region Merge on the region with the smaller one of the two adjacent Regions.

Note:

• All replicas of the two Regions to be merged must locate on the same set of TiKVs (It is ensured by PD scheduler).
• Newly split Regions won’t be merged within the period of time specified by split-merge-interval.
• Region Merge won’t happen within the period of time specified by split-merge-interval after PD starts or restarts.
• Region Merge won’t happen for two Regions that belong to different tables if namespace-classifier = table (default).

Configure Region Merge

Region Merge is enabled by default. You can use pd-ctl or the PD configuration file to configure Region Merge.

To enable Region Merge, set the following parameters to a non-zero value:

• max-merge-region-size
• max-merge-region-keys
• merge-schedule-limit

You can use split-merge-interval to control the interval between the split and merge operations.

For detailed descriptions on the above parameters, refer to PD Control.