Add more description about SHARD_ROW_ID_BITS (#21467) (#21597)

Author: ti-chi-bot

shard-row-id-bits.md

@@ -17,6 +17,22 @@ To mitigate the hot spot issue, you can configure `SHARD_ROW_ID_BITS`. The row I
 - `SHARD_ROW_ID_BITS = 6` indicates 64 shards
 - `SHARD_ROW_ID_BITS = 0` indicates the default 1 shard
 
+When you set `SHARD_ROW_ID_BITS = S`, the structure of `_tidb_rowid` is as follows:
+
+| Sign bit |  Shard bits | Auto-increment bits |
+|--------|--------|--------------|
+| 1 bit | `S` bits | `63-S` bits |
+
+- The values of the auto-increment bits are stored in TiKV and allocated sequentially. Each time a value is allocated, the next value is incremented by 1. The auto-increment bits ensure that the column values of `_tidb_rowid` are unique globally. When the value of the auto-increment bits is exhausted (that is, when the maximum value is reached), subsequent automatic allocations fail with the error `Failed to read auto-increment value from storage engine`.
+- The value range of `_tidb_rowid`: the maximum number of bits for the final generated value = shard bits + auto-increment bits, so the maximum value is `(2^63)-1`.
+
+> **Note:**
+>
+> Selection of shard bits (`S`):
+>
+> - Because the total bits of `_tidb_rowid` is 64, the number of shard bits affects the number of auto-increment bits: when the number of shard bits increases, the number of auto-increment bits decreases, and vice versa. Therefore, you need to balance the randomness of the auto-increment values and the available auto-increment space.
+> - The best practice is to set the shard bits to `log(2, x)`, where `x` is the number of TiKV nodes in the cluster. For example, if there are 16 TiKV nodes in a TiDB cluster, it is recommended to set the shard bits to `log(2, 16)`, which equals `4`. After all Regions are evenly scheduled to each TiKV node, the load of bulk writes can be evenly distributed to different TiKV nodes to maximize resource utilization.
+
 <CustomContent platform="tidb">
 
 For details on the usage, see [the Troubleshoot Hotspot Issues guide](/troubleshoot-hot-spot-issues.md#use-shard_row_id_bits-to-process-hotspots).