diff --git a/src/HttpServer.cc b/src/HttpServer.cc
index 000cd168..90fbb170 100644
--- a/src/HttpServer.cc
+++ b/src/HttpServer.cc
@@ -598,8 +598,7 @@ void Http2Handler::start_settings_timer() {
 
 int Http2Handler::fill_wb() {
   if (!data_pending_.empty()) {
-    auto n = std::min(wb_.wleft(), data_pending_.size());
-    wb_.write(data_pending_.data(), n);
+    auto n = wb_.write(data_pending_);
     if (n < data_pending_.size()) {
       data_pending_ = data_pending_.subspan(n);
       return 0;
@@ -620,9 +619,10 @@ int Http2Handler::fill_wb() {
     if (datalen == 0) {
       break;
     }
-    auto n = wb_.write(data, as_unsigned(datalen));
-    if (n < static_cast<decltype(n)>(datalen)) {
-      data_pending_ = {data + n, as_unsigned(datalen) - n};
+    auto chunk = std::span{data, as_unsigned(datalen)};
+    auto n = wb_.write(chunk);
+    if (n < chunk.size()) {
+      data_pending_ = chunk.subspan(n);
       break;
     }
   }
diff --git a/src/buffer.h b/src/buffer.h
index df42e8c0..b4f1e517 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -41,13 +41,12 @@ template <size_t N> struct Buffer {
   constexpr size_t wleft() const noexcept {
     return as_unsigned(&buf[N] - last);
   }
-  // Writes up to min(wleft(), |count|) bytes from buffer pointed by
-  // |src|.  Returns number of bytes written.
-  constexpr size_t write(const void *src, size_t count) {
-    count = std::min(count, wleft());
-    auto p = static_cast<const uint8_t *>(src);
-    last = std::ranges::copy_n(p, as_signed(count), last).out;
-    return count;
+  // Writes up to min(wleft(), |src|.size()) bytes from |src|.
+  // Returns number of bytes written.
+  constexpr size_t write(std::span<const uint8_t> src) {
+    src = src.first(std::min(src.size(), wleft()));
+    last = std::ranges::copy(src, last).out;
+    return src.size();
   }
   constexpr size_t write(size_t count) {
     count = std::min(count, wleft());
diff --git a/src/buffer_test.cc b/src/buffer_test.cc
index 8f2984c8..9d3f66fa 100644
--- a/src/buffer_test.cc
+++ b/src/buffer_test.cc
@@ -34,6 +34,8 @@
 
 #include "buffer.h"
 
+using namespace std::literals;
+
 namespace nghttp2 {
 
 namespace {
@@ -52,7 +54,7 @@ void test_buffer_write(void) {
   assert_size(0, ==, b.rleft());
   assert_size(16, ==, b.wleft());
 
-  b.write("012", 3);
+  b.write(as_uint8_span(std::span{"012"sv}));
 
   assert_size(3, ==, b.rleft());
   assert_size(13, ==, b.wleft());
@@ -64,7 +66,7 @@ void test_buffer_write(void) {
   assert_size(13, ==, b.wleft());
   assert_ptrdiff(3, ==, b.pos - std::ranges::begin(b.buf));
 
-  auto n = b.write("0123456789ABCDEF", 16);
+  auto n = b.write(as_uint8_span(std::span{"0123456789ABCDEF"sv}));
 
   assert_size(13, ==, n);
 
diff --git a/src/shrpx_memcached_connection.cc b/src/shrpx_memcached_connection.cc
index 8fdbcf83..63eddc64 100644
--- a/src/shrpx_memcached_connection.cc
+++ b/src/shrpx_memcached_connection.cc
@@ -703,7 +703,7 @@ void MemcachedConnection::make_request(MemcachedSendbuf *sendbuf,
     break;
   }
 
-  headbuf.write(req->key.c_str(), req->key.size());
+  headbuf.write(as_uint8_span(std::span{req->key}));
 
   sendbuf->send_value_left = req->value.size();
 }