diff --git a/internal/buffer/in_message.go b/internal/buffer/in_message.go
index c991b9a..2611f9d 100644
--- a/internal/buffer/in_message.go
+++ b/internal/buffer/in_message.go
@@ -16,16 +16,34 @@ package buffer
 
 import (
 	"errors"
+	"fmt"
 	"io"
+	"syscall"
 	"unsafe"
 
 	"github.com/jacobsa/fuse/internal/fusekernel"
 )
 
+// All requests read from the kernel, without data, are shorter than
+// this.
+const pageSize = 4096
+
+func init() {
+	// Confirm the page size.
+	if syscall.Getpagesize() != pageSize {
+		panic(fmt.Sprintf("Page size is unexpectedly %d", syscall.Getpagesize()))
+	}
+}
+
+// We size the buffer to have enough room for a fuse request plus data
+// associated with a write request.
+const bufSize = pageSize + MaxWriteSize
+
 // An incoming message from the kernel, including leading fusekernel.InHeader
 // struct. Provides storage for messages and convenient access to their
 // contents.
 type InMessage struct {
+	buf [bufSize]byte
 }
 
 // Initialize with the data read by a single call to r.Read. The first call to
diff --git a/internal/buffer/in_message_darwin.go b/internal/buffer/in_message_darwin.go
new file mode 100644
index 0000000..d1f6b9f
--- /dev/null
+++ b/internal/buffer/in_message_darwin.go
@@ -0,0 +1,20 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package buffer
+
+// The maximum fuse write request size that InMessage can acommodate.
+//
+// Experimentally, OS X appears to cap the size of writes to 1 MiB.
+const MaxWriteSize = 1 << 20