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store/tikv/coprocessor.go

@@ -236,6 +236,7 @@ func (r *copRanges) split(key []byte) (*copRanges, *copRanges) {
 	return r.slice(0, n), r.slice(n, r.len())
 }
 
+// rangesPerTask limits the length of the ranges slice sent in one copTask.
 const rangesPerTask = 25000
 
 func buildCopTasks(bo *Backoffer, cache *RegionCache, ranges *copRanges, desc bool, streaming bool) ([]*copTask, error) {
@@ -248,6 +249,8 @@ func buildCopTasks(bo *Backoffer, cache *RegionCache, ranges *copRanges, desc bo
 
 	var tasks []*copTask
 	appendTask := func(region RegionVerID, ranges *copRanges) {
+		// TiKV will return gRPC error if the message is too large. So we need to limit the length of the ranges slice
+		// to make sure the message can be sent successfully.
 		rLen := ranges.len()
 		for i := 0; i < rLen; {
 			nextI := mathutil.Min(i+rangesPerTask, rLen)