diff --git a/main-mshell.go b/main-mshell.go
index 0bdb4e779..e5120fd84 100644
--- a/main-mshell.go
+++ b/main-mshell.go
@@ -245,7 +245,7 @@ func handleRemote() {
 	defer cmd.Close()
 	startPacket := cmd.MakeCmdStartPacket()
 	sender.SendPacket(startPacket)
-	cmd.RunIOAndWait(sender)
+	cmd.RunIOAndWait(packetCh, sender)
 }
 
 func handleServer() {
diff --git a/pkg/packet/packet.go b/pkg/packet/packet.go
index 33989978e..fc9fcaa4c 100644
--- a/pkg/packet/packet.go
+++ b/pkg/packet/packet.go
@@ -28,6 +28,7 @@ const (
 	PingPacketStr      = "ping"
 	InitPacketStr      = "init"
 	DataPacketStr      = "data"
+	DataAckPacketStr   = "dataack"
 	CmdStartPacketStr  = "cmdstart"
 	CmdDonePacketStr   = "cmddone"
 	ResponsePacketStr  = "resp"
@@ -62,6 +63,7 @@ func init() {
 	TypeStrToFactory[RawPacketStr] = reflect.TypeOf(RawPacketType{})
 	TypeStrToFactory[InputPacketStr] = reflect.TypeOf(InputPacketType{})
 	TypeStrToFactory[DataPacketStr] = reflect.TypeOf(DataPacketType{})
+	TypeStrToFactory[DataAckPacketStr] = reflect.TypeOf(DataAckPacketType{})
 }
 
 func MakePacket(packetType string) (PacketType, error) {
@@ -128,6 +130,23 @@ func MakeDataPacket() *DataPacketType {
 	return &DataPacketType{Type: DataPacketStr}
 }
 
+type DataAckPacketType struct {
+	Type      string `json:"type"`
+	SessionId string `json:"sessionid,omitempty"`
+	CmdId     string `json:"cmdid,omitempty"`
+	FdNum     int    `json:"fdnum"`
+	AckLen    int    `json:"acklen"`
+	Error     string `json:"error"`
+}
+
+func (*DataAckPacketType) GetType() string {
+	return DataAckPacketStr
+}
+
+func MakeDataAckPacket() *DataAckPacketType {
+	return &DataAckPacketType{Type: DataAckPacketStr}
+}
+
 // InputData gets written to PTY directly
 // SigNum gets sent to process via a signal
 // WinSize, if set, will run TIOCSWINSZ to set size, and then send SIGWINCH
diff --git a/pkg/shexec/bufreader.go b/pkg/shexec/bufreader.go
new file mode 100644
index 000000000..73aefc7bf
--- /dev/null
+++ b/pkg/shexec/bufreader.go
@@ -0,0 +1,125 @@
+// Copyright 2022 Dashborg Inc
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package shexec
+
+import (
+	"io"
+	"os"
+	"sync"
+
+	"github.com/scripthaus-dev/mshell/pkg/packet"
+)
+
+type FdReader struct {
+	CVar      *sync.Cond
+	SessionId string
+	CmdId     string
+	FdNum     int
+	Fd        *os.File
+	BufSize   int
+	Closed    bool
+}
+
+func MakeFdReader(c *ShExecType, fd *os.File, fdNum int) *FdReader {
+	return &FdReader{
+		CVar:      sync.NewCond(&sync.Mutex{}),
+		SessionId: c.RunPacket.SessionId,
+		CmdId:     c.RunPacket.CmdId,
+		FdNum:     fdNum,
+		Fd:        fd,
+		BufSize:   0,
+	}
+}
+
+func (r *FdReader) Close() {
+	r.CVar.L.Lock()
+	defer r.CVar.L.Unlock()
+	if r.Closed {
+		return
+	}
+	if r.Fd != nil {
+		r.Fd.Close()
+	}
+	r.CVar.Broadcast()
+}
+
+func (r *FdReader) NotifyAck(ackLen int) {
+	r.CVar.L.Lock()
+	defer r.CVar.L.Unlock()
+	r.BufSize -= ackLen
+	if r.BufSize < 0 {
+		r.BufSize = 0
+	}
+	r.CVar.Broadcast()
+}
+
+// returns (success)
+func (r *FdReader) WriteWait(sender *packet.PacketSender, data []byte, isEof bool) bool {
+	if len(data) == 0 {
+		return true
+	}
+	r.CVar.L.Lock()
+	defer r.CVar.L.Unlock()
+	for {
+		bufAvail := ReadBufSize - r.BufSize
+		if r.Closed {
+			return false
+		}
+		if bufAvail == 0 {
+			r.CVar.Wait()
+			continue
+		}
+		writeLen := min(bufAvail, len(data))
+		pk := r.MakeDataPacket(data[0:writeLen], nil)
+		sender.SendPacket(pk)
+		r.BufSize += writeLen
+		data = data[writeLen:]
+		if len(data) == 0 {
+			return true
+		}
+		r.CVar.Wait()
+	}
+}
+
+func min(v1 int, v2 int) int {
+	if v1 <= v2 {
+		return v1
+	}
+	return v2
+}
+
+func (r *FdReader) MakeDataPacket(data []byte, err error) *packet.DataPacketType {
+	pk := packet.MakeDataPacket()
+	pk.SessionId = r.SessionId
+	pk.CmdId = r.CmdId
+	pk.FdNum = r.FdNum
+	pk.Data = string(data)
+	if err != nil {
+		pk.Error = err.Error()
+	}
+	return pk
+}
+
+func (r *FdReader) ReadLoop(wg *sync.WaitGroup, sender *packet.PacketSender) {
+	defer r.Close()
+	defer wg.Done()
+	buf := make([]byte, 4096)
+	for {
+		nr, err := r.Fd.Read(buf)
+		if nr > 0 || err == io.EOF {
+			isOpen := r.WriteWait(sender, buf[0:nr], (err == io.EOF))
+			if !isOpen {
+				return
+			}
+		}
+		if err != nil {
+			errPk := r.MakeDataPacket(nil, err)
+			sender.SendPacket(errPk)
+			return
+		}
+	}
+}
diff --git a/pkg/shexec/bufwriter.go b/pkg/shexec/bufwriter.go
new file mode 100644
index 000000000..f51a06049
--- /dev/null
+++ b/pkg/shexec/bufwriter.go
@@ -0,0 +1,115 @@
+// Copyright 2022 Dashborg Inc
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package shexec
+
+import (
+	"fmt"
+	"os"
+	"sync"
+
+	"github.com/scripthaus-dev/mshell/pkg/packet"
+)
+
+type FdWriter struct {
+	CVar      *sync.Cond
+	SessionId string
+	CmdId     string
+	FdNum     int
+	Buffer    []byte
+	Fd        *os.File
+	Eof       bool
+	Closed    bool
+}
+
+func MakeFdWriter(c *ShExecType, fd *os.File, fdNum int) *FdWriter {
+	return &FdWriter{
+		CVar:      sync.NewCond(&sync.Mutex{}),
+		Fd:        fd,
+		SessionId: c.RunPacket.SessionId,
+		CmdId:     c.RunPacket.CmdId,
+		FdNum:     fdNum,
+	}
+}
+
+func (w *FdWriter) Close() {
+	w.CVar.L.Lock()
+	defer w.CVar.L.Unlock()
+	if w.Closed {
+		return
+	}
+	w.Closed = true
+	if w.Fd != nil {
+		w.Fd.Close()
+	}
+	w.Buffer = nil
+	w.CVar.Broadcast()
+}
+
+func (w *FdWriter) WaitForData() ([]byte, bool) {
+	w.CVar.L.Lock()
+	defer w.CVar.L.Unlock()
+	for {
+		if len(w.Buffer) > 0 || w.Eof || w.Closed {
+			toWrite := w.Buffer
+			w.Buffer = nil
+			return toWrite, w.Eof
+		}
+		w.CVar.Wait()
+	}
+}
+
+func (w *FdWriter) MakeDataAckPacket(ackLen int, err error) *packet.DataAckPacketType {
+	ack := packet.MakeDataAckPacket()
+	ack.SessionId = w.SessionId
+	ack.CmdId = w.CmdId
+	ack.FdNum = w.FdNum
+	ack.AckLen = ackLen
+	if err != nil {
+		ack.Error = err.Error()
+	}
+	return ack
+}
+
+func (w *FdWriter) AddData(data []byte, eof bool) error {
+	w.CVar.L.Lock()
+	defer w.CVar.L.Unlock()
+	if w.Closed {
+		return fmt.Errorf("write to closed file")
+	}
+	if len(data) > 0 {
+		if len(data)+len(w.Buffer) > WriteBufSize {
+			return fmt.Errorf("write exceeds buffer size")
+		}
+		w.Buffer = append(w.Buffer, data...)
+	}
+	if eof {
+		w.Eof = true
+	}
+	w.CVar.Broadcast()
+	return nil
+}
+
+func (w *FdWriter) WriteLoop(sender *packet.PacketSender) {
+	defer w.Close()
+	for {
+		data, isEof := w.WaitForData()
+		if w.Closed {
+			return
+		}
+		if len(data) > 0 {
+			nw, err := w.Fd.Write(data)
+			ack := w.MakeDataAckPacket(nw, err)
+			sender.SendPacket(ack)
+			if err != nil {
+				return
+			}
+		}
+		if isEof {
+			return
+		}
+	}
+}
diff --git a/pkg/shexec/shexec.go b/pkg/shexec/shexec.go
index e2cd6a8ec..3afedc296 100644
--- a/pkg/shexec/shexec.go
+++ b/pkg/shexec/shexec.go
@@ -26,36 +26,43 @@ const DefaultRows = 25
 const DefaultCols = 80
 const MaxRows = 1024
 const MaxCols = 1024
+const ReadBufSize = 128 * 1024
+const WriteBufSize = 128 * 1024
 
 type ShExecType struct {
+	Lock            *sync.Mutex
 	StartTs         time.Time
 	RunPacket       *packet.RunPacketType
 	FileNames       *base.CommandFileNames
 	Cmd             *exec.Cmd
 	CmdPty          *os.File
-	FdReaders       map[int]*os.File
-	FdWriters       map[int]*os.File
-	CloseAfterStart []*os.File
+	FdReaders       map[int]*FdReader // synchronized
+	FdWriters       map[int]*FdWriter // synchronized
+	CloseAfterStart []*os.File        // synchronized
 }
 
 func MakeShExec(pk *packet.RunPacketType) *ShExecType {
 	return &ShExecType{
+		Lock:      &sync.Mutex{},
 		StartTs:   time.Now(),
 		RunPacket: pk,
-		FdReaders: make(map[int]*os.File),
-		FdWriters: make(map[int]*os.File),
+		FdReaders: make(map[int]*FdReader),
+		FdWriters: make(map[int]*FdWriter),
 	}
 }
 
 func (c *ShExecType) Close() {
+	c.Lock.Lock()
+	defer c.Lock.Unlock()
+
 	if c.CmdPty != nil {
 		c.CmdPty.Close()
 	}
 	for _, fd := range c.FdReaders {
 		fd.Close()
 	}
-	for _, fd := range c.FdWriters {
-		fd.Close()
+	for _, fw := range c.FdWriters {
+		fw.Close()
 	}
 	for _, fd := range c.CloseAfterStart {
 		fd.Close()
@@ -221,7 +228,9 @@ func (cmd *ShExecType) makeReaderPipe(fdNum int) (*os.File, error) {
 	if err != nil {
 		return nil, err
 	}
-	cmd.FdReaders[fdNum] = pr
+	cmd.Lock.Lock()
+	defer cmd.Lock.Unlock()
+	cmd.FdReaders[fdNum] = MakeFdReader(cmd, pr, fdNum)
 	cmd.CloseAfterStart = append(cmd.CloseAfterStart, pw)
 	return pw, nil
 }
@@ -232,51 +241,81 @@ func (cmd *ShExecType) makeWriterPipe(fdNum int) (*os.File, error) {
 	if err != nil {
 		return nil, err
 	}
-	cmd.FdWriters[fdNum] = pw
+	cmd.Lock.Lock()
+	defer cmd.Lock.Unlock()
+	cmd.FdWriters[fdNum] = MakeFdWriter(cmd, pw, fdNum)
 	cmd.CloseAfterStart = append(cmd.CloseAfterStart, pr)
 	return pr, nil
 }
 
-func (cmd *ShExecType) MakeDataPacket(fdNum int, data []byte) *packet.DataPacketType {
-	pk := packet.MakeDataPacket()
-	pk.SessionId = cmd.RunPacket.SessionId
-	pk.CmdId = cmd.RunPacket.CmdId
-	pk.FdNum = fdNum
-	pk.Data = string(data)
-	return pk
+func (cmd *ShExecType) MakeDataAckPacket(fdNum int, ackLen int, err error) *packet.DataAckPacketType {
+	ack := packet.MakeDataAckPacket()
+	ack.SessionId = cmd.RunPacket.SessionId
+	ack.CmdId = cmd.RunPacket.CmdId
+	ack.FdNum = fdNum
+	ack.AckLen = ackLen
+	if err != nil {
+		ack.Error = err.Error()
+	}
+	return ack
 }
 
-func (cmd *ShExecType) runReadLoop(wg *sync.WaitGroup, fdNum int, fd *os.File, sender *packet.PacketSender) {
-	go func() {
-		defer fd.Close()
-		defer wg.Done()
-		buf := make([]byte, 4096)
-		for {
-			nr, err := fd.Read(buf)
-			pk := cmd.MakeDataPacket(fdNum, buf[0:nr])
-			if err == io.EOF {
-				pk.Eof = true
-				sender.SendPacket(pk)
-				break
-			} else if err != nil {
-				pk.Error = err.Error()
-				sender.SendPacket(pk)
-				break
-			} else {
-				sender.SendPacket(pk)
-			}
+func (cmd *ShExecType) launchWriters(sender *packet.PacketSender) {
+	cmd.Lock.Lock()
+	defer cmd.Lock.Unlock()
+	for _, fw := range cmd.FdWriters {
+		go fw.WriteLoop(sender)
+	}
+}
+
+func (cmd *ShExecType) writeDataPacket(dataPacket *packet.DataPacketType) error {
+	cmd.Lock.Lock()
+	defer cmd.Lock.Unlock()
+	fw := cmd.FdWriters[dataPacket.FdNum]
+	if fw == nil {
+		// add a closed FdWriter as a placeholder so we only send one error
+		fw := MakeFdWriter(cmd, nil, dataPacket.FdNum)
+		fw.Close()
+		cmd.FdWriters[dataPacket.FdNum] = fw
+		return fmt.Errorf("write to closed file")
+	}
+	err := fw.AddData([]byte(dataPacket.Data), dataPacket.Eof)
+	if err != nil {
+		fw.Close()
+		return err
+	}
+	return nil
+}
+
+func (cmd *ShExecType) runMainWriteLoop(packetCh chan packet.PacketType, sender *packet.PacketSender) {
+	for pk := range packetCh {
+		if pk.GetType() != packet.DataPacketStr {
+			// other packets are ignored
+			continue
 		}
-	}()
+		dataPacket := pk.(*packet.DataPacketType)
+		err := cmd.writeDataPacket(dataPacket)
+		if err != nil {
+			errPacket := cmd.MakeDataAckPacket(dataPacket.FdNum, 0, err)
+			sender.SendPacket(errPacket)
+		}
+	}
 }
 
-func (cmd *ShExecType) RunIOAndWait(sender *packet.PacketSender) {
-	var wg sync.WaitGroup
+func (cmd *ShExecType) launchReaders(wg *sync.WaitGroup, sender *packet.PacketSender) {
+	cmd.Lock.Lock()
+	defer cmd.Lock.Unlock()
 	wg.Add(len(cmd.FdReaders))
-	go func() {
-		for fdNum, fd := range cmd.FdReaders {
-			cmd.runReadLoop(&wg, fdNum, fd, sender)
-		}
-	}()
+	for _, fr := range cmd.FdReaders {
+		go fr.ReadLoop(wg, sender)
+	}
+}
+
+func (cmd *ShExecType) RunIOAndWait(packetCh chan packet.PacketType, sender *packet.PacketSender) {
+	var wg sync.WaitGroup
+	cmd.launchReaders(&wg, sender)
+	cmd.launchWriters(sender)
+	go cmd.runMainWriteLoop(packetCh, sender)
 	donePacket := cmd.WaitForCommand()
 	wg.Wait()
 	sender.SendPacket(donePacket)