Index: lib/libc/sys/aio_fsync.2
===================================================================
--- lib/libc/sys/aio_fsync.2
+++ lib/libc/sys/aio_fsync.2
@@ -49,11 +49,15 @@
 .Pp
 The
 .Fa op
-argument can only be set to
+argument can be set to
 .Dv O_SYNC
 to cause all currently queued I/O operations to be completed
 as if by a call to
-.Xr fsync 2 .
+.Xr fsync 2 ,
+or
+.Dv O_DSYNC
+for
+.Xr fdatasync 2 .
 .Pp
 If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it,
 then the enqueued operation is submitted at a priority equal to that
@@ -176,3 +180,7 @@
 .Fn aio_fsync
 system call first appeared in
 .Fx 7.0 .
+The
+.Dv O_DSYNC
+option appeared in
+.Fx 13.0 .
Index: sys/kern/vfs_aio.c
===================================================================
--- sys/kern/vfs_aio.c
+++ sys/kern/vfs_aio.c
@@ -715,10 +715,10 @@
 
 /*
  * Move all data to a permanent storage device.  This code
- * simulates the fsync syscall.
+ * simulates the fsync and fdatasync syscalls.
  */
 static int
-aio_fsync_vnode(struct thread *td, struct vnode *vp)
+aio_fsync_vnode(struct thread *td, struct vnode *vp, bool fullsync)
 {
 	struct mount *mp;
 	int error;
@@ -731,7 +731,7 @@
 		vm_object_page_clean(vp->v_object, 0, 0, 0);
 		VM_OBJECT_WUNLOCK(vp->v_object);
 	}
-	error = VOP_FSYNC(vp, MNT_WAIT, td);
+	error = fullsync ? VOP_FSYNC(vp, MNT_WAIT, td) : VOP_FDATASYNC(vp, td);
 
 	VOP_UNLOCK(vp);
 	vn_finished_write(mp);
@@ -841,13 +841,16 @@
 	struct ucred *td_savedcred = td->td_ucred;
 	struct file *fp = job->fd_file;
 	int error = 0;
+	bool fullsync;
 
-	KASSERT(job->uaiocb.aio_lio_opcode == LIO_SYNC,
+	KASSERT(job->uaiocb.aio_lio_opcode == LIO_SYNC ||
+		job->uaiocb.aio_lio_opcode == LIO_DSYNC,
 	    ("%s: opcode %d", __func__, job->uaiocb.aio_lio_opcode));
 
+	fullsync = job->uaiocb.aio_lio_opcode == LIO_SYNC;
 	td->td_ucred = job->cred;
 	if (fp->f_vnode != NULL)
-		error = aio_fsync_vnode(td, fp->f_vnode);
+		error = aio_fsync_vnode(td, fp->f_vnode, fullsync);
 	td->td_ucred = td_savedcred;
 	if (error)
 		aio_complete(job, -1, error);
@@ -1533,6 +1536,7 @@
 		error = fget_read(td, fd, &cap_pread_rights, &fp);
 		break;
 	case LIO_SYNC:
+	case LIO_DSYNC:
 		error = fget(td, fd, &cap_fsync_rights, &fp);
 		break;
 	case LIO_MLOCK:
@@ -1550,7 +1554,7 @@
 		return (error);
 	}
 
-	if (opcode == LIO_SYNC && fp->f_vnode == NULL) {
+	if ((opcode == LIO_SYNC || opcode == LIO_DSYNC) && fp->f_vnode == NULL) {
 		error = EINVAL;
 		goto aqueue_fail;
 	}
@@ -1722,6 +1726,7 @@
 		error = 0;
 		break;
 	case LIO_SYNC:
+	case LIO_DSYNC:
 		AIO_LOCK(ki);
 		TAILQ_FOREACH(job2, &ki->kaio_jobqueue, plist) {
 			if (job2->fd_file == job->fd_file &&
@@ -2467,10 +2472,12 @@
 kern_aio_fsync(struct thread *td, int op, struct aiocb *ujob,
     struct aiocb_ops *ops)
 {
+	int listop;
 
-	if (op != O_SYNC) /* XXX lack of O_DSYNC */
+	if (op != O_SYNC && op != O_DSYNC)
 		return (EINVAL);
-	return (aio_aqueue(td, ujob, NULL, LIO_SYNC, ops));
+	listop = op == O_SYNC ? LIO_SYNC : LIO_DSYNC;
+	return (aio_aqueue(td, ujob, NULL, listop, ops));
 }
 
 int
Index: sys/sys/aio.h
===================================================================
--- sys/sys/aio.h
+++ sys/sys/aio.h
@@ -45,6 +45,7 @@
 #ifdef _KERNEL
 #define	LIO_SYNC		0x3
 #define	LIO_MLOCK		0x4
+#define	LIO_DSYNC		0x5
 #endif
 
 /*
Index: sys/sys/fcntl.h
===================================================================
--- sys/sys/fcntl.h
+++ sys/sys/fcntl.h
@@ -138,8 +138,10 @@
 #define	O_BENEATH	0x00400000	/* Fail if not under cwd */
 #endif
 
+#define	O_DSYNC		0x00800000	/* POSIX O_DSYNC */
+
 /*
- * XXX missing O_DSYNC, O_RSYNC.
+ * XXX missing O_RSYNC.
  */
 
 #ifdef _KERNEL
Index: tests/sys/aio/aio_test.c
===================================================================
--- tests/sys/aio/aio_test.c
+++ tests/sys/aio/aio_test.c
@@ -1055,7 +1055,7 @@
 	ATF_REQUIRE_MSG(fd != -1, "open failed: %s", strerror(errno));
 	unlink(FILE_PATHNAME);
 
-	/* aio_fsync should return EINVAL unless op is O_SYNC */
+	/* aio_fsync should return EINVAL unless op is O_SYNC or O_DSYNC */
 	memset(&iocb, 0, sizeof(iocb));
 	iocb.aio_fildes = fd;
 	ATF_CHECK_EQ(-1, aio_fsync(666, &iocb));