From 9bbf7fe2ab87026ad91b2c238ad6945bd7c71c7d Mon Sep 17 00:00:00 2001
From: Morgan Tocker <tocker@gmail.com>
Date: Wed, 6 Jan 2021 12:38:05 -0700
Subject: [PATCH] Remove GC Mode since it is no longer effective

---
 session/session.go               |  4 +---
 sessionctx/variable/sysvar.go    |  1 -
 sessionctx/variable/tidb_vars.go | 12 +++++-------
 3 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/session/session.go b/session/session.go
index 82d88151c9aac..964dec04e6257 100644
--- a/session/session.go
+++ b/session/session.go
@@ -103,7 +103,6 @@ var gcVariableComments = map[string]string{
 	variable.TiDBGCLifetime:     "All versions within life time will not be collected by GC, at least 10m, in Go format.",
 	variable.TiDBGCConcurrency:  "How many goroutines used to do GC parallel, [1, 128], default 2",
 	variable.TiDBGCEnable:       "Current GC enable status",
-	variable.TiDBGCMode:         "Mode of GC, \"central\" or \"distributed\"",
 	tiKVGCAutoConcurrency:       "Let TiDB pick the concurrency automatically. If set false, tikv_gc_concurrency will be used",
 	variable.TiDBGCScanLockMode: "Mode of scanning locks, \"physical\" or \"legacy\"",
 }
@@ -113,7 +112,6 @@ var gcVariableMap = map[string]string{
 	variable.TiDBGCLifetime:     "tikv_gc_life_time",
 	variable.TiDBGCConcurrency:  "tikv_gc_concurrency",
 	variable.TiDBGCEnable:       "tikv_gc_enable",
-	variable.TiDBGCMode:         "tikv_gc_mode",
 	variable.TiDBGCScanLockMode: "tikv_gc_scan_lock_mode",
 }
 
@@ -1011,7 +1009,7 @@ func (s *session) getExecRet(ctx sessionctx.Context, sql string) (string, error)
 
 func (s *session) varFromTiDBTable(name string) bool {
 	switch name {
-	case variable.TiDBGCConcurrency, variable.TiDBGCEnable, variable.TiDBGCRunInterval, variable.TiDBGCLifetime, variable.TiDBGCMode, variable.TiDBGCScanLockMode:
+	case variable.TiDBGCConcurrency, variable.TiDBGCEnable, variable.TiDBGCRunInterval, variable.TiDBGCLifetime, variable.TiDBGCScanLockMode:
 		return true
 	}
 	return false
diff --git a/sessionctx/variable/sysvar.go b/sessionctx/variable/sysvar.go
index a99f3e4757934..bbfd712fda3e4 100644
--- a/sessionctx/variable/sysvar.go
+++ b/sessionctx/variable/sysvar.go
@@ -1220,7 +1220,6 @@ var defaultSysVars = []*SysVar{
 	{Scope: ScopeGlobal, Name: TiDBGCRunInterval, Value: "10m0s", Type: TypeDuration, MinValue: int64(time.Minute * 10), MaxValue: math.MaxInt64},
 	{Scope: ScopeGlobal, Name: TiDBGCLifetime, Value: "10m0s", Type: TypeDuration, MinValue: int64(time.Minute * 10), MaxValue: math.MaxInt64},
 	{Scope: ScopeGlobal, Name: TiDBGCConcurrency, Value: "-1", Type: TypeInt, MinValue: 1, MaxValue: 128, AllowAutoValue: true},
-	{Scope: ScopeGlobal, Name: TiDBGCMode, Value: "DISTRIBUTED", Type: TypeEnum, PossibleValues: []string{"DISTRIBUTED", "CENTRAL"}},
 	{Scope: ScopeGlobal, Name: TiDBGCScanLockMode, Value: "PHYSICAL", Type: TypeEnum, PossibleValues: []string{"PHYSICAL", "LEGACY"}},
 }
 
diff --git a/sessionctx/variable/tidb_vars.go b/sessionctx/variable/tidb_vars.go
index 6f2c9cf48cf40..45139b4e8e46d 100644
--- a/sessionctx/variable/tidb_vars.go
+++ b/sessionctx/variable/tidb_vars.go
@@ -521,17 +521,15 @@ const (
 // TiDB vars that have only global scope
 
 const (
-	// TiDBGCEnable description
+	// TiDBGCEnable turns garbage collection on or OFF
 	TiDBGCEnable = "tidb_gc_enable"
-	// TiDBGCRunInterval description
+	// TiDBGCRunInterval sets the interval that GC runs
 	TiDBGCRunInterval = "tidb_gc_run_interval"
-	// TiDBGCLifetime description
+	// TiDBGCLifetime sets the retention window of older versions
 	TiDBGCLifetime = "tidb_gc_life_time"
-	// TiDBGCConcurrency description
+	// TiDBGCConcurrency sets the concurrency of garbage collection. -1 = AUTO value
 	TiDBGCConcurrency = "tidb_gc_concurrency"
-	// TiDBGCMode description
-	TiDBGCMode = "tidb_gc_mode"
-	// TiDBGCScanLockMode description
+	// TiDBGCScanLockMode enables the green GC feature (default)
 	TiDBGCScanLockMode = "tidb_gc_scan_lock_mode"
 )