# TiKV Region Splits
+ TiKV Region Leaders are subject to relocation and rebalancing among TiKV Stores
+ TiKV Region Leaders count on every TiKV Store is balanced by `Placement Driver` eventually
+ At the beginning, splitting TiKV Regions reside on the same TiKV Store
+ TiKV Region split is controlled by following TiKV parameters
  + **`coprocessor.region-split-size`** is 96 MB by default before version 8.5 and is 256 MB from version 8.5
  + **`coprocessor.region-max-size`** is 3/2 of **`coprocessor.region-split-size`** by default

![`smart`](TiKVRegionSplit_en_001_20231121.png)
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As we learned so far, all kinds of modification result in data addition to TiKV Region. Despite the history data kept by `UPDATE/DELETE` which will be recycled during garbage collection eventually, when a TiKV Region reaches a certain size limit, it splits.

When a TiKV Region is initialized, the default size is set to 96 MB. If the size exceeds 144 MB due to DML changes, the region will be split. The split threshold is determined by the TiKV configuration parameter `coprocessor.region-max-size` after TiDB version 6.1.0. When the size of a TiKV Region reaches the value specified by `coprocessor.region-max-size`, it will trigger the split. The default value of `coprocessor.region-max-size` is calculated as `(coprocessor.region-split-size / 2) x 3`, and the default value of `coprocessor.region-split-size` is 96 MB.

The data in a table with clustered index for primary key is stored in TiKV in the order of the primary key. Therefore, the primary key in the table can be seen as the TiKV key (with the appropriate prefix), eliminating the need to store the primary key in the TiKV value entry.

`coprocessor.region-split-size`: The size of the newly split TiKV Region.
`coprocessor.region-max-size`: The maximum size of a TiKV Region. When the value is exceeded, the TiKV Region splits into multiple TiKV Regions.

Please enroll the training course "TiDB Cluster Performance Tuning" for more details about how `Placement Driver` balances the TiKV Region Leaders among TiKV nodes.