diff --git a/sys/fs/nfs/nfs_commonsubs.c b/sys/fs/nfs/nfs_commonsubs.c
index 817d8928409..20f5618c202 100644
--- a/sys/fs/nfs/nfs_commonsubs.c
+++ b/sys/fs/nfs/nfs_commonsubs.c
@@ -179,7 +179,7 @@ struct nfsv4_opflag nfsv4_opflag[NFSV42_NOPS] = {
 	{ 0, 1, 1, 1, LK_EXCLUSIVE, 1, 0 },		/* Allocate */
 	{ 2, 1, 1, 0, LK_SHARED, 1, 0 },		/* Copy */
 	{ 0, 0, 0, 0, LK_EXCLUSIVE, 1, 1 },		/* Copy Notify */
-	{ 0, 0, 0, 0, LK_EXCLUSIVE, 1, 1 },		/* Deallocate */
+	{ 0, 2, 1, 1, LK_EXCLUSIVE, 1, 0 },		/* Deallocate */
 	{ 0, 1, 0, 0, LK_SHARED, 1, 0 },		/* IO Advise */
 	{ 0, 1, 0, 0, LK_EXCLUSIVE, 1, 0 },		/* Layout Error */
 	{ 0, 1, 0, 0, LK_EXCLUSIVE, 1, 0 },		/* Layout Stats */
@@ -219,7 +219,7 @@ static struct nfsrv_lughash	*nfsgroupnamehash;
 static int nfs_bigreply[NFSV42_NPROCS] = { 0, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
-    1, 0, 0, 1, 0 };
+    1, 0, 0, 1, 0, 0, 0 };
 
 /* local functions */
 static int nfsrv_skipace(struct nfsrv_descript *nd, int *acesizep);
@@ -303,6 +303,8 @@ static struct {
 	{ NFSV4OP_REMOVEXATTR, 2, "Rmxattr", 7, },
 	{ NFSV4OP_LISTXATTRS, 2, "Listxattr", 9, },
 	{ NFSV4OP_BINDCONNTOSESS, 1, "BindConSess", 11, },
+	{ NFSV4OP_LOOKUP, 5, "LookupOpen", 10, },
+	{ NFSV4OP_DEALLOCATE, 2, "Deallocate", 10, },
 };
 
 /*
@@ -311,7 +313,7 @@ static struct {
 static int nfs_bigrequest[NFSV42_NPROCS] = {
 	0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0
+	0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0
 };
 
 /*
@@ -434,7 +436,8 @@ nfscl_reqstart(struct nfsrv_descript *nd, int procnum, struct nfsmount *nmp,
 				 * attributes, so we can load the name cache.
 				 */
 				if (procnum == NFSPROC_LOOKUP ||
-				    procnum == NFSPROC_LOOKUPP)
+				    procnum == NFSPROC_LOOKUPP ||
+				    procnum == NFSPROC_LOOKUPOPEN)
 					NFSGETATTR_ATTRBIT(&attrbits);
 				else {
 					NFSWCCATTR_ATTRBIT(&attrbits);
diff --git a/sys/fs/nfs/nfs_var.h b/sys/fs/nfs/nfs_var.h
index 9cbaeae361a..b4c21c6f702 100644
--- a/sys/fs/nfs/nfs_var.h
+++ b/sys/fs/nfs/nfs_var.h
@@ -465,7 +465,7 @@ int nfsrpc_setattr(vnode_t, struct vattr *, NFSACL_T *, struct ucred *,
     NFSPROC_T *, struct nfsvattr *, int *, void *);
 int nfsrpc_lookup(vnode_t, char *, int, struct ucred *, NFSPROC_T *,
     struct nfsvattr *, struct nfsvattr *, struct nfsfh **, int *, int *,
-    void *);
+    void *, uint32_t);
 int nfsrpc_readlink(vnode_t, struct uio *, struct ucred *,
     NFSPROC_T *, struct nfsvattr *, int *, void *);
 int nfsrpc_read(vnode_t, struct uio *, struct ucred *, NFSPROC_T *,
@@ -624,6 +624,8 @@ void nfscl_reclaimnode(vnode_t);
 void nfscl_newnode(vnode_t);
 void nfscl_delegmodtime(vnode_t);
 void nfscl_deleggetmodtime(vnode_t, struct timespec *);
+int nfscl_trydelegreturn(struct nfscldeleg *, struct ucred *,
+    struct nfsmount *, NFSPROC_T *);
 int nfscl_tryclose(struct nfsclopen *, struct ucred *,
     struct nfsmount *, NFSPROC_T *);
 void nfscl_cleanup(NFSPROC_T *);
diff --git a/sys/fs/nfs/nfsport.h b/sys/fs/nfs/nfsport.h
index cb82666ab39..64ba4d48b3f 100644
--- a/sys/fs/nfs/nfsport.h
+++ b/sys/fs/nfs/nfsport.h
@@ -415,10 +415,16 @@
 /* BindConnectionToSession, done by the krpc for a new connection. */
 #define	NFSPROC_BINDCONNTOSESS	65
 
+/* Do a Lookup+Open for "oneopenown". */
+#define	NFSPROC_LOOKUPOPEN	66
+
+/* Do an NFSv4.2 Deallocate. */
+#define	NFSPROC_DEALLOCATE	67
+
 /*
  * Must be defined as one higher than the last NFSv4.2 Proc# above.
  */
-#define	NFSV42_NPROCS		66
+#define	NFSV42_NPROCS		68
 
 #endif	/* NFS_V3NPROCS */
 
@@ -447,7 +453,7 @@ struct nfsstatsv1 {
 	uint64_t	readlink_bios;
 	uint64_t	biocache_readdirs;
 	uint64_t	readdir_bios;
-	uint64_t	rpccnt[NFSV42_NPROCS + 14];
+	uint64_t	rpccnt[NFSV42_NPROCS + 12];
 	uint64_t	rpcretries;
 	uint64_t	srvrpccnt[NFSV42_NOPS + NFSV4OP_FAKENOPS + 15];
 	uint64_t	reserved_0;
@@ -512,7 +518,7 @@ struct nfsstatsov1 {
 	uint64_t	readlink_bios;
 	uint64_t	biocache_readdirs;
 	uint64_t	readdir_bios;
-	uint64_t	rpccnt[NFSV42_NPROCS + 3];
+	uint64_t	rpccnt[NFSV42_NPROCS + 1];
 	uint64_t	rpcretries;
 	uint64_t	srvrpccnt[NFSV42_PURENOPS + NFSV4OP_FAKENOPS];
 	uint64_t	reserved_0;
diff --git a/sys/fs/nfs/nfsproto.h b/sys/fs/nfs/nfsproto.h
index 13e14615480..7ac7cdc3bac 100644
--- a/sys/fs/nfs/nfsproto.h
+++ b/sys/fs/nfs/nfsproto.h
@@ -396,10 +396,16 @@
 /* BindConnectionToSession, done by the krpc for a new connection. */
 #define	NFSPROC_BINDCONNTOSESS	65
 
+/* Do a Lookup+Open for "oneopenown". */
+#define	NFSPROC_LOOKUPOPEN	66
+
+/* Do an NFSv4.2 Deallocate. */
+#define	NFSPROC_DEALLOCATE	67
+
 /*
  * Must be defined as one higher than the last NFSv4.2 Proc# above.
  */
-#define	NFSV42_NPROCS		66
+#define	NFSV42_NPROCS		68
 
 #endif	/* NFS_V3NPROCS */
 
diff --git a/sys/fs/nfsclient/nfs_clrpcops.c b/sys/fs/nfsclient/nfs_clrpcops.c
index 74803e255aa..de4030876bf 100644
--- a/sys/fs/nfsclient/nfs_clrpcops.c
+++ b/sys/fs/nfsclient/nfs_clrpcops.c
@@ -1382,15 +1382,20 @@ nfsrpc_setattrrpc(vnode_t vp, struct vattr *vap,
 int
 nfsrpc_lookup(vnode_t dvp, char *name, int len, struct ucred *cred,
     NFSPROC_T *p, struct nfsvattr *dnap, struct nfsvattr *nap,
-    struct nfsfh **nfhpp, int *attrflagp, int *dattrflagp, void *stuff)
+    struct nfsfh **nfhpp, int *attrflagp, int *dattrflagp, void *stuff,
+    uint32_t openmode)
 {
-	u_int32_t *tl;
+	uint32_t deleg, rflags, *tl;
 	struct nfsrv_descript nfsd, *nd = &nfsd;
 	struct nfsmount *nmp;
 	struct nfsnode *np;
 	struct nfsfh *nfhp;
 	nfsattrbit_t attrbits;
-	int error = 0, lookupp = 0;
+	int error = 0, lookupp = 0, newone, ret, retop;
+	uint8_t own[NFSV4CL_LOCKNAMELEN];
+	struct nfsclopen *op;
+	struct nfscldeleg *ndp;
+	nfsv4stateid_t stateid;
 
 	*attrflagp = 0;
 	*dattrflagp = 0;
@@ -1415,7 +1420,11 @@ nfsrpc_lookup(vnode_t dvp, char *name, int len, struct ucred *cred,
 	if (NFSHASNFSV4(nmp) && len == 2 &&
 		name[0] == '.' && name[1] == '.') {
 		lookupp = 1;
+		openmode = 0;
 		NFSCL_REQSTART(nd, NFSPROC_LOOKUPP, dvp);
+	} else if (openmode != 0) {
+		NFSCL_REQSTART(nd, NFSPROC_LOOKUPOPEN, dvp);
+		nfsm_strtom(nd, name, len);
 	} else {
 		NFSCL_REQSTART(nd, NFSPROC_LOOKUP, dvp);
 		(void) nfsm_strtom(nd, name, len);
@@ -1426,10 +1435,36 @@ nfsrpc_lookup(vnode_t dvp, char *name, int len, struct ucred *cred,
 		*tl++ = txdr_unsigned(NFSV4OP_GETFH);
 		*tl = txdr_unsigned(NFSV4OP_GETATTR);
 		(void) nfsrv_putattrbit(nd, &attrbits);
+		if (openmode != 0) {
+			/* Test for a VREG file. */
+			NFSZERO_ATTRBIT(&attrbits);
+			NFSSETBIT_ATTRBIT(&attrbits, NFSATTRBIT_TYPE);
+			NFSM_BUILD(tl, uint32_t *, NFSX_UNSIGNED);
+			*tl = txdr_unsigned(NFSV4OP_VERIFY);
+			nfsrv_putattrbit(nd, &attrbits);
+			NFSM_BUILD(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+			*tl++ = txdr_unsigned(NFSX_UNSIGNED);
+			*tl = vtonfsv34_type(VREG);
+
+			/* Attempt the Open for VREG. */
+			nfscl_filllockowner(NULL, own, F_POSIX);
+			NFSM_BUILD(tl, uint32_t *, 6 * NFSX_UNSIGNED);
+			*tl++ = txdr_unsigned(NFSV4OP_OPEN);
+			*tl++ = 0;		/* seqid, ignored. */
+			*tl++ = txdr_unsigned(openmode);
+			*tl++ = txdr_unsigned(NFSV4OPEN_DENYNONE);
+			*tl++ = 0;		/* ClientID, ignored. */
+			*tl = 0;
+			nfsm_strtom(nd, own, NFSV4CL_LOCKNAMELEN);
+			NFSM_BUILD(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+			*tl++ = txdr_unsigned(NFSV4OPEN_NOCREATE);
+			*tl = txdr_unsigned(NFSV4OPEN_CLAIMFH);
+		}
 	}
 	error = nfscl_request(nd, dvp, p, cred, stuff);
 	if (error)
 		return (error);
+	ndp = NULL;
 	if (nd->nd_repstat) {
 		/*
 		 * When an NFSv4 Lookupp returns ENOENT, it means that
@@ -1453,6 +1488,33 @@ nfsrpc_lookup(vnode_t dvp, char *name, int len, struct ucred *cred,
 			error = nfsm_loadattr(nd, dnap);
 			if (error == 0)
 				*dattrflagp = 1;
+			else
+				goto nfsmout;
+		}
+		/* Check Lookup operation reply status. */
+		if (openmode != 0 && (nd->nd_flag & ND_NOMOREDATA) == 0) {
+			NFSM_DISSECT(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+			if (*++tl != 0)
+				goto nfsmout;
+		}
+		/* Look for GetFH reply. */
+		if (openmode != 0 && (nd->nd_flag & ND_NOMOREDATA) == 0) {
+			NFSM_DISSECT(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+			if (*++tl != 0)
+				goto nfsmout;
+			error = nfsm_getfh(nd, nfhpp);
+			if (error)
+				goto nfsmout;
+		}
+		/* Look for Getattr reply. */
+		if (openmode != 0 && (nd->nd_flag & ND_NOMOREDATA) == 0) {
+			NFSM_DISSECT(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+			if (*++tl != 0)
+				goto nfsmout;
+			error = nfscl_postop_attr(nd, nap, attrflagp, stuff);
+			if (error == 0)
+				/* Successfully got Lookup done. */
+				nd->nd_repstat = 0;
 		}
 		goto nfsmout;
 	}
@@ -1470,12 +1532,84 @@ nfsrpc_lookup(vnode_t dvp, char *name, int len, struct ucred *cred,
 		goto nfsmout;
 
 	error = nfscl_postop_attr(nd, nap, attrflagp, stuff);
+	if (openmode != 0 && error == 0) {
+		NFSM_DISSECT(tl, uint32_t *, NFSX_STATEID +
+		    10 * NFSX_UNSIGNED);
+		tl += 4;	/* Skip over Verify+Open status. */
+		stateid.seqid = *tl++;
+		stateid.other[0] = *tl++;
+		stateid.other[1] = *tl++;
+		stateid.other[2] = *tl;
+		rflags = fxdr_unsigned(uint32_t, *(tl + 6));
+		error = nfsrv_getattrbits(nd, &attrbits, NULL, NULL);
+		if (error != 0)
+			goto nfsmout;
+		NFSM_DISSECT(tl, uint32_t *, NFSX_UNSIGNED);
+		deleg = fxdr_unsigned(uint32_t, *tl);
+		if (deleg == NFSV4OPEN_DELEGATEREAD ||
+		    deleg == NFSV4OPEN_DELEGATEWRITE) {
+			/*
+			 * Just need to fill in the fields used by
+			 * nfscl_trydelegreturn().
+			 * Mark the mount point as acquiring
+			 * delegations, so NFSPROC_LOOKUPOPEN will
+			 * no longer be done.
+			 */
+			NFSLOCKMNT(nmp);
+			nmp->nm_privflag |= NFSMNTP_DELEGISSUED;
+			NFSUNLOCKMNT(nmp);
+			ndp = malloc(sizeof(struct nfscldeleg) +
+			    (*nfhpp)->nfh_len, M_NFSCLDELEG, M_WAITOK);
+			ndp->nfsdl_fhlen = (*nfhpp)->nfh_len;
+			NFSBCOPY((*nfhpp)->nfh_fh, ndp->nfsdl_fh,
+			    ndp->nfsdl_fhlen);
+			newnfs_copyincred(cred, &ndp->nfsdl_cred);
+			NFSM_DISSECT(tl, uint32_t *, NFSX_STATEID);
+			ndp->nfsdl_stateid.seqid = *tl++;
+			ndp->nfsdl_stateid.other[0] = *tl++;
+			ndp->nfsdl_stateid.other[1] = *tl++;
+			ndp->nfsdl_stateid.other[2] = *tl++;
+		} else if (deleg != NFSV4OPEN_DELEGATENONE) {
+			error = NFSERR_BADXDR;
+			goto nfsmout;
+		}
+		ret = nfscl_open(dvp, (*nfhpp)->nfh_fh, (*nfhpp)->nfh_len,
+		    openmode, 0, cred, p, NULL, &op, &newone, &retop, 1);
+		if (ret != 0)
+			goto nfsmout;
+		if (newone != 0) {
+			op->nfso_stateid.seqid = stateid.seqid;
+			op->nfso_stateid.other[0] = stateid.other[0];
+			op->nfso_stateid.other[1] = stateid.other[1];
+			op->nfso_stateid.other[2] = stateid.other[2];
+			op->nfso_mode = openmode;
+		} else {
+			op->nfso_stateid.seqid = stateid.seqid;
+			if (retop == NFSCLOPEN_DOOPEN)
+				op->nfso_mode |= openmode;
+		}
+		if ((rflags & NFSV4OPEN_LOCKTYPEPOSIX) != 0 ||
+		    nfscl_assumeposixlocks)
+			op->nfso_posixlock = 1;
+		else
+			op->nfso_posixlock = 0;
+		nfscl_openrelease(nmp, op, 0, 0);
+		if (ndp != NULL) {
+			/*
+			 * Since we do not have the vnode, we
+			 * cannot invalidate cached attributes.
+			 * Just return the delegation.
+			 */
+			nfscl_trydelegreturn(ndp, cred, nmp, p);
+		}
+	}
 	if ((nd->nd_flag & ND_NFSV3) && !error)
 		error = nfscl_postop_attr(nd, dnap, dattrflagp, stuff);
 nfsmout:
 	m_freem(nd->nd_mrep);
 	if (!error && nd->nd_repstat)
 		error = nd->nd_repstat;
+	free(ndp, M_NFSCLDELEG);
 	return (error);
 }
 
diff --git a/sys/fs/nfsclient/nfs_clstate.c b/sys/fs/nfsclient/nfs_clstate.c
index bb2c78a72ed..8ec5b80489f 100644
--- a/sys/fs/nfsclient/nfs_clstate.c
+++ b/sys/fs/nfsclient/nfs_clstate.c
@@ -162,8 +162,6 @@ static int nfscl_recalldeleg(struct nfsclclient *, struct nfsmount *,
     vnode_t *);
 static void nfscl_freeopenowner(struct nfsclowner *, int);
 static void nfscl_cleandeleg(struct nfscldeleg *);
-static int nfscl_trydelegreturn(struct nfscldeleg *, struct ucred *,
-    struct nfsmount *, NFSPROC_T *);
 static void nfscl_emptylockowner(struct nfscllockowner *,
     struct nfscllockownerfhhead *);
 static void nfscl_mergeflayouts(struct nfsclflayouthead *,
@@ -4463,7 +4461,7 @@ nfscl_trylock(struct nfsmount *nmp, vnode_t vp, u_int8_t *fhp,
  * retrying while NFSERR_DELAY. Also, try system credentials, if the passed in
  * credentials fail.
  */
-static int
+int
 nfscl_trydelegreturn(struct nfscldeleg *dp, struct ucred *cred,
     struct nfsmount *nmp, NFSPROC_T *p)
 {
diff --git a/sys/fs/nfsclient/nfs_clvnops.c b/sys/fs/nfsclient/nfs_clvnops.c
index 72d9eac8e96..39340692563 100644
--- a/sys/fs/nfsclient/nfs_clvnops.c
+++ b/sys/fs/nfsclient/nfs_clvnops.c
@@ -1164,6 +1164,7 @@ nfs_lookup(struct vop_lookup_args *ap)
 	struct nfsvattr dnfsva, nfsva;
 	struct vattr vattr;
 	struct timespec nctime;
+	uint32_t openmode;
 
 	*vpp = NULLVP;
 	if ((flags & ISLASTCN) && (mp->mnt_flag & MNT_RDONLY) &&
@@ -1265,11 +1266,30 @@ nfs_lookup(struct vop_lookup_args *ap)
 		cache_purge_negative(dvp);
 	}
 
+	/*
+	 * If this an NFSv4.1/4.2 mount using the "oneopenown" mount
+	 * option, it is possible to do the Open operation in the same
+	 * compound as Lookup, so long as delegations are not being
+	 * issued.  This saves doing a separate RPC for Open.
+	 */
+	openmode = 0;
+	NFSLOCKMNT(nmp);
+	if (NFSHASNFSV4N(nmp) && NFSHASONEOPENOWN(nmp) &&
+	    (nmp->nm_privflag & NFSMNTP_DELEGISSUED) == 0 &&
+	    (!NFSMNT_RDONLY(mp) || (flags & OPENWRITE) == 0) &&
+	    (flags & (ISLASTCN | ISOPEN)) == (ISLASTCN | ISOPEN)) {
+		if ((flags & OPENREAD) != 0)
+			openmode |= NFSV4OPEN_ACCESSREAD;
+		if ((flags & OPENWRITE) != 0)
+			openmode |= NFSV4OPEN_ACCESSWRITE;
+	}
+	NFSUNLOCKMNT(nmp);
+
 	newvp = NULLVP;
 	NFSINCRGLOBAL(nfsstatsv1.lookupcache_misses);
 	error = nfsrpc_lookup(dvp, cnp->cn_nameptr, cnp->cn_namelen,
 	    cnp->cn_cred, td, &dnfsva, &nfsva, &nfhp, &attrflag, &dattrflag,
-	    NULL);
+	    NULL, openmode);
 	if (dattrflag)
 		(void) nfscl_loadattrcache(&dvp, &dnfsva, NULL, NULL, 0, 1);
 	if (error) {
@@ -1577,7 +1597,7 @@ nfs_mknodrpc(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
 			(void) nfsrpc_lookup(dvp, cnp->cn_nameptr,
 			    cnp->cn_namelen, cnp->cn_cred, cnp->cn_thread,
 			    &dnfsva, &nfsva, &nfhp, &attrflag, &dattrflag,
-			    NULL);
+			    NULL, 0);
 		if (nfhp)
 			error = nfscl_nget(dvp->v_mount, dvp, nfhp, cnp,
 			    cnp->cn_thread, &np, NULL, LK_EXCLUSIVE);
@@ -1693,7 +1713,7 @@ nfs_create(struct vop_create_args *ap)
 			(void) nfsrpc_lookup(dvp, cnp->cn_nameptr,
 			    cnp->cn_namelen, cnp->cn_cred, cnp->cn_thread,
 			    &dnfsva, &nfsva, &nfhp, &attrflag, &dattrflag,
-			    NULL);
+			    NULL, 0);
 		if (nfhp != NULL)
 			error = nfscl_nget(dvp->v_mount, dvp, nfhp, cnp,
 			    cnp->cn_thread, &np, NULL, LK_EXCLUSIVE);
@@ -2611,7 +2631,7 @@ nfs_lookitup(struct vnode *dvp, char *name, int len, struct ucred *cred,
 	u_int hash;
 
 	error = nfsrpc_lookup(dvp, name, len, cred, td, &dnfsva, &nfsva,
-	    &nfhp, &attrflag, &dattrflag, NULL);
+	    &nfhp, &attrflag, &dattrflag, NULL, 0);
 	if (dattrflag)
 		(void) nfscl_loadattrcache(&dvp, &dnfsva, NULL, NULL, 0, 1);
 	if (npp && !error) {