diff --git a/src/h2load.cc b/src/h2load.cc
index 64609843..4dc68f97 100644
--- a/src/h2load.cc
+++ b/src/h2load.cc
@@ -318,7 +318,8 @@ void client_request_timeout_cb(struct ev_loop *loop, ev_timer *w, int revents) {
 } // namespace
 
 Client::Client(uint32_t id, Worker *worker, size_t req_todo)
-    : cstat{},
+    : wb(&worker->mcpool),
+      cstat{},
       worker(worker),
       ssl(nullptr),
       next_addr(config.addrs),
@@ -910,6 +911,10 @@ int Client::on_read(const uint8_t *data, size_t len) {
 }
 
 int Client::on_write() {
+  if (wb.rleft() >= BACKOFF_WRITE_BUFFER_THRES) {
+    return 0;
+  }
+
   if (session->on_write() != 0) {
     return -1;
   }
@@ -943,28 +948,32 @@ int Client::read_clear() {
 }
 
 int Client::write_clear() {
+  std::array<struct iovec, 2> iov;
+
   for (;;) {
-    if (wb.rleft() > 0) {
-      ssize_t nwrite;
-      while ((nwrite = write(fd, wb.pos, wb.rleft())) == -1 && errno == EINTR)
-        ;
-      if (nwrite == -1) {
-        if (errno == EAGAIN || errno == EWOULDBLOCK) {
-          ev_io_start(worker->loop, &wev);
-          return 0;
-        }
-        return -1;
-      }
-      wb.drain(nwrite);
-      continue;
-    }
-    wb.reset();
     if (on_write() != 0) {
       return -1;
     }
-    if (wb.rleft() == 0) {
+
+    auto iovcnt = wb.riovec(iov.data(), iov.size());
+
+    if (iovcnt == 0) {
       break;
     }
+
+    ssize_t nwrite;
+    while ((nwrite = writev(fd, iov.data(), iovcnt)) == -1 && errno == EINTR)
+      ;
+
+    if (nwrite == -1) {
+      if (errno == EAGAIN || errno == EWOULDBLOCK) {
+        ev_io_start(worker->loop, &wev);
+        return 0;
+      }
+      return -1;
+    }
+
+    wb.drain(nwrite);
   }
 
   ev_io_stop(worker->loop, &wev);
@@ -1057,35 +1066,36 @@ int Client::read_tls() {
 int Client::write_tls() {
   ERR_clear_error();
 
+  struct iovec iov;
+
   for (;;) {
-    if (wb.rleft() > 0) {
-      auto rv = SSL_write(ssl, wb.pos, wb.rleft());
-
-      if (rv <= 0) {
-        auto err = SSL_get_error(ssl, rv);
-        switch (err) {
-        case SSL_ERROR_WANT_READ:
-          // renegotiation started
-          return -1;
-        case SSL_ERROR_WANT_WRITE:
-          ev_io_start(worker->loop, &wev);
-          return 0;
-        default:
-          return -1;
-        }
-      }
-
-      wb.drain(rv);
-
-      continue;
-    }
-    wb.reset();
     if (on_write() != 0) {
       return -1;
     }
-    if (wb.rleft() == 0) {
+
+    auto iovcnt = wb.riovec(&iov, 1);
+
+    if (iovcnt == 0) {
       break;
     }
+
+    auto rv = SSL_write(ssl, iov.iov_base, iov.iov_len);
+
+    if (rv <= 0) {
+      auto err = SSL_get_error(ssl, rv);
+      switch (err) {
+      case SSL_ERROR_WANT_READ:
+        // renegotiation started
+        return -1;
+      case SSL_ERROR_WANT_WRITE:
+        ev_io_start(worker->loop, &wev);
+        return 0;
+      default:
+        return -1;
+      }
+    }
+
+    wb.drain(rv);
   }
 
   ev_io_stop(worker->loop, &wev);
diff --git a/src/h2load.h b/src/h2load.h
index 9ba4e484..39737805 100644
--- a/src/h2load.h
+++ b/src/h2load.h
@@ -50,13 +50,15 @@
 #include <openssl/ssl.h>
 
 #include "http2.h"
-#include "buffer.h"
+#include "memchunk.h"
 #include "template.h"
 
 using namespace nghttp2;
 
 namespace h2load {
 
+constexpr auto BACKOFF_WRITE_BUFFER_THRES = 16_k;
+
 class Session;
 struct Worker;
 
@@ -225,6 +227,7 @@ struct Sampling {
 };
 
 struct Worker {
+  MemchunkPool mcpool;
   Stats stats;
   Sampling request_times_smp;
   Sampling client_smp;
@@ -267,6 +270,7 @@ struct Stream {
 };
 
 struct Client {
+  DefaultMemchunks wb;
   std::unordered_map<int32_t, Stream> streams;
   ClientStat cstat;
   std::unique_ptr<Session> session;
@@ -293,7 +297,6 @@ struct Client {
   // The client id per worker
   uint32_t id;
   int fd;
-  Buffer<64_k> wb;
   ev_timer conn_active_watcher;
   ev_timer conn_inactivity_watcher;
   std::string selected_proto;
diff --git a/src/h2load_http1_session.cc b/src/h2load_http1_session.cc
index 968e1e32..09446092 100644
--- a/src/h2load_http1_session.cc
+++ b/src/h2load_http1_session.cc
@@ -174,7 +174,7 @@ int Http1Session::submit_request() {
   auto req_stat = client_->get_req_stat(stream_req_counter_);
 
   client_->record_request_time(req_stat);
-  client_->wb.write(req.c_str(), req.size());
+  client_->wb.append(req);
 
   // TODO try read some data here
 
@@ -226,8 +226,12 @@ int Http1Session::on_write() {
     auto req_stat = client_->get_req_stat(stream_req_counter_);
     auto &wb = client_->wb;
 
+    // TODO unfortunately, wb has no interface to use with read(2)
+    // family functions.
+    std::array<uint8_t, 16_k> buf;
+
     ssize_t nread;
-    while ((nread = pread(config->data_fd, wb.last, wb.wleft(),
+    while ((nread = pread(config->data_fd, buf.data(), buf.size(),
                           req_stat->data_offset)) == -1 &&
            errno == EINTR)
       ;
@@ -238,7 +242,7 @@ int Http1Session::on_write() {
 
     req_stat->data_offset += nread;
 
-    wb.write(nread);
+    wb.append(buf.data(), nread);
 
     if (client_->worker->config->verbose) {
       std::cout << "[send " << nread << " byte(s)]" << std::endl;
diff --git a/src/h2load_http2_session.cc b/src/h2load_http2_session.cc
index 71f1d75c..0b780128 100644
--- a/src/h2load_http2_session.cc
+++ b/src/h2load_http2_session.cc
@@ -169,11 +169,11 @@ ssize_t send_callback(nghttp2_session *session, const uint8_t *data,
   auto client = static_cast<Client *>(user_data);
   auto &wb = client->wb;
 
-  if (wb.wleft() == 0) {
+  if (wb.rleft() >= BACKOFF_WRITE_BUFFER_THRES) {
     return NGHTTP2_ERR_WOULDBLOCK;
   }
 
-  return wb.write(data, length);
+  return wb.append(data, length);
 }
 } // namespace
 
diff --git a/src/h2load_spdy_session.cc b/src/h2load_spdy_session.cc
index 40c29d55..54083809 100644
--- a/src/h2load_spdy_session.cc
+++ b/src/h2load_spdy_session.cc
@@ -112,11 +112,11 @@ ssize_t send_callback(spdylay_session *session, const uint8_t *data,
   auto client = static_cast<Client *>(user_data);
   auto &wb = client->wb;
 
-  if (wb.wleft() == 0) {
+  if (wb.rleft() >= BACKOFF_WRITE_BUFFER_THRES) {
     return SPDYLAY_ERR_DEFERRED;
   }
 
-  return wb.write(data, length);
+  return wb.append(data, length);
 }
 } // namespace