ddl: do not fill TiKV block cache for reorg coprocessor scans

pkg/ddl/backfilling_test.go

@@ -404,6 +404,17 @@ func assertDistSQLCtxEqual(t *testing.T, expected *distsqlctx.DistSQLContext, ac
 	require.Equal(t, errctx.NewContextWithLevels(expected.ErrCtx.LevelMap(), expected.WarnHandler), actual.ErrCtx)
 }
 
+func TestReorgDistSQLCtxNotFillCache(t *testing.T) {
+	// Reorg coprocessor scans are single-pass bulk reads and must not pollute the TiKV
+	// block cache, so NotFillCache is always set on the reorg DistSQLContext.
+	ctx := newDefaultReorgDistSQLCtx(&mock.Client{}, contextutil.NewStaticWarnHandler(0))
+	require.True(t, ctx.NotFillCache)
+
+	withMeta, err := newReorgDistSQLCtxWithReorgMeta(&mock.Client{}, &model.DDLReorgMeta{}, contextutil.NewStaticWarnHandler(0))
+	require.NoError(t, err)
+	require.True(t, withMeta.NotFillCache)
+}
+
 func TestValidateAndFillRanges(t *testing.T) {
 	mkRange := func(start, end string) kv.KeyRange {
 		return kv.KeyRange{StartKey: []byte(start), EndKey: []byte(end)}

pkg/ddl/backfilling_txn_executor.go

@@ -154,8 +154,13 @@ func newDefaultReorgDistSQLCtx(kvClient kv.Client, warnHandler contextutil.WarnA
 	var execDetails execdetails.SyncExecDetails
 	var cpuUsages ppcpuusage.SQLCPUUsages
 	return &distsqlctx.DistSQLContext{
-		WarnHandler:                          warnHandler,
-		Client:                               kvClient,
+		WarnHandler: warnHandler,
+		Client:      kvClient,
+		// Reorg scans are single-pass bulk reads over the whole table/index range: the blocks
+		// they touch are never re-read by the reorg, so caching them brings no benefit and can
+		// evict the working set that concurrent foreground traffic relies on. Skip filling the
+		// block cache for all reorg coprocessor scans.
+		NotFillCache:                         true,
 		EnableChunkRPC:                       true,
 		EnabledRateLimitAction:               vardef.DefTiDBEnableRateLimitAction,
 		KVVars:                               tikvstore.NewVariables(&sqlKiller.Signal),