server: add experimental flag for using shared buffer in transacton write

server/config/config.go

@@ -177,6 +177,10 @@ type ServerConfig struct {
 	// Currently all etcd memory gets mlocked, but in future the flag can
 	// be refined to mlock in-use area of bbolt only.
 	ExperimentalMemoryMlock bool `json:"experimental-memory-mlock"`
+
+	// ExperimentalTxnModeWriteWithSharedBuffer enable write transaction to use
+	// a shared buffer in its readonly check operations.
+	ExperimentalTxnModeWriteWithSharedBuffer bool `json:"experimental-txn-mode-write-with-shared-buffer"`
 }
 
 // VerifyBootstrap sanity-checks the initial config for bootstrap case

server/embed/config.go

@@ -355,6 +355,9 @@ type Config struct {
 	// Currently all etcd memory gets mlocked, but in future the flag can
 	// be refined to mlock in-use area of bbolt only.
 	ExperimentalMemoryMlock bool `json:"experimental-memory-mlock"`
+
+	// ExperimentalTxnModeWriteWithSharedBuffer enables write transaction to use a shared buffer in its readonly check operations.
+	ExperimentalTxnModeWriteWithSharedBuffer bool `json:"experimental-txn-mode-write-with-shared-buffer"`
 }
 
 // configYAML holds the config suitable for yaml parsing
@@ -446,6 +449,7 @@ func NewConfig() *Config {
 
 		ExperimentalDowngradeCheckTime:           DefaultDowngradeCheckTime,
 		ExperimentalMemoryMlock:                  false,
+		ExperimentalTxnModeWriteWithSharedBuffer: true,
 	}
 	cfg.InitialCluster = cfg.InitialClusterFromName(cfg.Name)
 	return cfg

server/embed/etcd.go

@@ -213,6 +213,7 @@ func StartEtcd(inCfg *Config) (e *Etcd, err error) {
 		DowngradeCheckTime:                       cfg.ExperimentalDowngradeCheckTime,
 		WarningApplyDuration:                     cfg.ExperimentalWarningApplyDuration,
 		ExperimentalMemoryMlock:                  cfg.ExperimentalMemoryMlock,
+		ExperimentalTxnModeWriteWithSharedBuffer: cfg.ExperimentalTxnModeWriteWithSharedBuffer,
 	}
 	print(e.cfg.logger, *cfg, srvcfg, memberInitialized)
 	if e.Server, err = etcdserver.NewServer(srvcfg); err != nil {

server/etcdmain/config.go

@@ -267,6 +267,7 @@ func newConfig() *config {
 	fs.DurationVar(&cfg.ec.ExperimentalDowngradeCheckTime, "experimental-downgrade-check-time", cfg.ec.ExperimentalDowngradeCheckTime, "Duration of time between two downgrade status check.")
 	fs.DurationVar(&cfg.ec.ExperimentalWarningApplyDuration, "experimental-warning-apply-duration", cfg.ec.ExperimentalWarningApplyDuration, "Time duration after which a warning is generated if request takes more time.")
 	fs.BoolVar(&cfg.ec.ExperimentalMemoryMlock, "experimental-memory-mlock", cfg.ec.ExperimentalMemoryMlock, "Enable to enforce etcd pages (in particular bbolt) to stay in RAM.")
+	fs.BoolVar(&cfg.ec.ExperimentalTxnModeWriteWithSharedBuffer, "experimental-txn-mode-write-with-shared-buffer", true, "Enable the write transaction to use a shared buffer in its readonly check operations.")
 
 	// unsafe
 	fs.BoolVar(&cfg.ec.UnsafeNoFsync, "unsafe-no-fsync", false, "Disables fsync, unsafe, will cause data loss.")

server/etcdmain/help.go

@@ -220,6 +220,8 @@ Experimental feature:
     Duration of periodical watch progress notification.
   --experimental-warning-apply-duration '100ms'
 	Warning is generated if requests take more than this duration.
+  --experimental-txn-mode-write-with-shared-buffer 'true'
+    Enable the write transaction to use a shared buffer in its readonly check operations.
 
 Unsafe feature:
   --force-new-cluster 'false'

server/etcdserver/apply.go

@@ -438,7 +438,7 @@ func (a *applierV3backend) Txn(ctx context.Context, rt *pb.TxnRequest) (*pb.TxnR
 	// When the transaction contains write operations, we use ReadTx instead of
 	// ConcurrentReadTx to avoid extra overhead of copying buffer.
 	var txn mvcc.TxnWrite
-	if isWrite {
+	if isWrite && a.s.Cfg.ExperimentalTxnModeWriteWithSharedBuffer {
 		txn = mvcc.NewReadOnlyTxnWrite(a.s.KV().Read(mvcc.SharedBufReadTxMode, trace))
 	} else {
 		txn = mvcc.NewReadOnlyTxnWrite(a.s.KV().Read(mvcc.ConcurrentReadTxMode, trace))