diff --git a/src/backend/storage/ipc/dsm_impl.c b/src/backend/storage/ipc/dsm_impl.c
index 0819641..1c3aa9a 100644
--- a/src/backend/storage/ipc/dsm_impl.c
+++ b/src/backend/storage/ipc/dsm_impl.c
@@ -368,7 +368,7 @@ dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
 
 	/* Map it. */
 	address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
-				   MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+				   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
 	if (address == MAP_FAILED)
 	{
 		int			save_errno;
@@ -960,7 +960,7 @@ dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
 
 	/* Map it. */
 	address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
-				   MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+				   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
 	if (address == MAP_FAILED)
 	{
 		int			save_errno;
diff --git a/src/include/portability/mem.h b/src/include/portability/mem.h
index a0bde5e..8eeac24 100644
--- a/src/include/portability/mem.h
+++ b/src/include/portability/mem.h
@@ -30,6 +30,12 @@
 #define MAP_HASSEMAPHORE		0
 #endif
 
+/* BSD-derived systems use the MAP_NOSYNC flag to prevent dirty mmap(2) pages
+ * from being gratuitously flushed to disk. */
+#ifndef MAP_NOSYNC
+#define MAP_NOSYNC			0
+#endif
+
 #define PG_MMAP_FLAGS			(MAP_SHARED|MAP_ANONYMOUS|MAP_HASSEMAPHORE)
 
 /* Some really old systems don't define MAP_FAILED. */