docs: add the design of Enhance Split Table Stability and Usability

docs/design/2021-10-22-Enhance-Split-Table-Stability.md

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+# Proposal: Enhance Split Table Stability and Usability
+
+- Author(s): [hzh](https://github.com/hzh0425)
+- Discussion PR: https://github.com/tikv/pd/issues/4095
+- Tracking Issue: https://github.com/pingcap/tidb/issues/29034
+
+## Table of Contents
+
+* [Introduction](#introduction)
+* [Motivation or Background](#motivation-or-background)
+* [Detailed Design](#detailed-design)
+* [Test Design](#test-design)
+* [Follow-up work](#Follow-up work)
+
+## Introduction
+
+Currently, the split table(regions) in tidb is constructed by following 2 steps:
+
+1. Notify tikv to split regions
+2. Notify pd to scatter regions
+
+But this will let the time cost for the whole splitting table become unstable and rather time-consuming.
+
+This proposal aims to split and scatter regions in pd directly
+
+## Motivation or Background
+
+Split table region : split a region into multi regions by split-keys
+
+Scatter table regions: regions are still in the original stores after split. Scatter is to load balance regions and evenly distribute regions to other stores
+
+However, before this proposal, TIDB and some tools were inconsistent when calling PD API -- split and scatter regions.
+
+Our purpose is to add a new  PD API -- SplitAndScatterRegions(), so that TIDB and other tools can easily call this API uniformly
+
+## Detailed Design
+
+### kvproto: pdpb.proto
+
+add a new interface SplitAndScatterRegions in pdpb.proto
+
+```
+rpc SplitAndScatterRegions(SplitAndScatterRegionsRequest) returns (SplitAndScatterRegionsResponse) {}
+```
+
+### PD: Grpc_Service, client.go
+
+Implement SplitAndScatterRegions in grpc_service and client
+
+grpc_service can directly call the original implementation splitRegions and scatterregions to achieve simultaneous split and scatter
+
+client can
+
+### TIDB
+
+1.add new interface SplitAndScatterRegions in kv/kv.go/SplittableStore
+
+```
+SplitAndScatterRegions(ctx context.Context, splitKeys [][]byte, tableID *int64) (regionID []uint64, err error)
+```
+
+this interface will be implement by client-go
+
+2.use new api SplitAndScatterRegions in executor/split.go
+
+### client-go
+
+Implement SplitAndScatterRegions in tikv/split_regions.go
+
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+
+## Test Design
+
+I will test the stability and performance by controlling variables under different parameter conditions
+
+For example:
+
+- Split a empty table into 32 regions / 400 regions ....
+- Split a table that contains data into 32 regions / 400 regions ....
+- Split a table which dose not contain primary key into 32 regions / 400 regions ....
+- ...............
+
+I will record the time spent in these cases
+
+
+
+## Follow-up work
+
+Enhance split region statement
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