diff --git a/share/man/man4/geom.4 b/share/man/man4/geom.4
--- a/share/man/man4/geom.4
+++ b/share/man/man4/geom.4
@@ -418,6 +418,50 @@
 a separate section below.
 .El
 .Pp
+Because the non-blocking allocation of
+.Vt "struct bio"
+is frequently required,
+.Va "biozone" ,
+the default
+.Xr uma 9
+zone for
+.Vt "struct bio"
+has some items preallocated and reserved by
+.Xr uma_prealloc 9
+and
+.Xr uma_zone_reserve 9 .
+Only the non-blocking
+.Vt "struct bio"
+allocation requests can use the reserved items, including cloning.
+The number of the reserved items are configurable by loader tunable
+.Va kern.geom.reserved_new_bios .
+.Pp
+An I/O request originator may allocate
+.Vt "struct bio"
+out of its own
+.Xr uma 9
+zone instead of
+.Va "biozone" .
+This is useful when an isolated
+.Xr uma 9
+zone is desirable due to the nature of the I/O requests.
+The write requests by the VM swap pager is a good example.
+A swap write operation is meant to make some free pages, which, in turn,
+should not involve any blocking memory allocation including
+.Vt "struct bio" .
+The VM swap pager hence has the even higher demand for the
+.Vt "struct bio"
+reservation, and has its own
+.Xr uma 9
+zone for that.
+Its reservation is configurable by loader tunable
+.Va vm.swap_reserved_new_bios .
+Other kernel subsystems may have their own
+.Xr uma 9
+zones for
+.Vt "struct bio" .
+Please refer to their respective man pages for the detail.
+.Pp
 (Stay tuned while the author rests his brain and fingers: more to come.)
 .Sh DIAGNOSTICS
 Several flags are provided for tracing
diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -1128,11 +1128,15 @@
 MLINKS+=g_attach.9 g_detach.9
 MLINKS+=g_bio.9 bio.9 \
 	g_bio.9 g_alloc_bio.9 \
+	g_bio.9 g_alloc_bio_uz.9 \
 	g_bio.9 g_clone_bio.9 \
+	g_bio.9 g_ctor_bio.9 \
 	g_bio.9 g_destroy_bio.9 \
 	g_bio.9 g_duplicate_bio.9 \
 	g_bio.9 g_format_bio.9 \
+	g_bio.9 g_io_new_uz.9 \
 	g_bio.9 g_new_bio.9 \
+	g_bio.9 g_new_bio_uz.9 \
 	g_bio.9 g_print_bio.9 \
 	g_bio.9 g_reset_bio.9
 MLINKS+=g_consumer.9 g_destroy_consumer.9 \
diff --git a/share/man/man9/g_bio.9 b/share/man/man9/g_bio.9
--- a/share/man/man9/g_bio.9
+++ b/share/man/man9/g_bio.9
@@ -27,11 +27,16 @@
 .Os
 .Sh NAME
 .Nm g_new_bio ,
+.Nm g_new_bio_uz ,
+.Nm g_alloc_bio ,
+.Nm g_alloc_bio_uz ,
 .Nm g_clone_bio ,
 .Nm g_destroy_bio ,
 .Nm g_format_bio ,
 .Nm g_print_bio ,
-.Nm g_reset_bio
+.Nm g_reset_bio ,
+.Nm g_io_new_uz ,
+.Nm g_ctor_bio
 .Nd "GEOM bio controlling functions"
 .Sh SYNOPSIS
 .In sys/bio.h
@@ -39,8 +44,12 @@
 .Ft "struct bio *"
 .Fn g_new_bio void
 .Ft "struct bio *"
+.Fn g_new_bio_uz "uma_zone_t uz"
+.Ft "struct bio *"
 .Fn g_alloc_bio void
 .Ft "struct bio *"
+.Fn g_alloc_bio_uz "uma_zone_t uz"
+.Ft "struct bio *"
 .Fn g_clone_bio "struct bio *bp"
 .Ft "struct bio *"
 .Fn g_duplicate_bio "struct bio *bp"
@@ -55,6 +64,10 @@
 .Fc
 .Ft void
 .Fn g_reset_bio "struct bio *bp"
+.Ft uma_zone_t
+.Fn g_io_new_uz "const char *name"
+.Ft int
+.Fn g_ctor_bio "void *mem" "int size" "void *arg" "int flags"
 .Sh DESCRIPTION
 A
 .Vt "struct bio"
@@ -143,20 +156,42 @@
 .It Va bio_parent
 Pointer to parent
 .Vt bio .
+.It Va bio_uz
+.Xr uma 9
+zone from which this
+.Vt bio
+structure is allocated.
 .El
 .Pp
 The
 .Fn g_new_bio
-function allocates a new, empty
+and
+.Fn g_new_bio_uz
+functions allocate a new, empty
 .Vt bio
-structure.
+structure from
+.Va biozone ,
+the default
+.Xr uma 9
+zone for
+.Vt bio
+and the supplied one, respectively.
 .Pp
-.Fn g_alloc_bio
+.Fn g_alloc_bio ,
+.Fn g_alloc_bio_uz
 - same as
-.Fn g_new_bio ,
-but always succeeds (allocates bio with the
+.Fn g_new_bio
+and
+.Fn g_new_bio_uz ,
+respectively, but always succeed at the possible cost of blocking (allocate
+.Vt bio
+with the
 .Dv M_WAITOK
-malloc flag).
+.Xr uma_zalloc 9
+flag).
+NB these functions do not allocate from the reserves even if the
+.Xr uma 9
+zone has them, for the sake of saving the reserves.
 .Pp
 The
 .Fn g_clone_bio
@@ -169,7 +204,9 @@
 .Va bio_length ,
 .Va bio_offset ,
 .Va bio_data ,
-.Va bio_attribute .
+.Va bio_attribute
+and
+.Va bio_uz .
 The field
 .Va bio_parent
 in the clone points to the passed
@@ -179,6 +216,12 @@
 in the passed
 .Vt bio
 is incremented.
+The same
+.Xr uma 9
+zone as the argument
+.Vt bio
+is used to allocate the cloned
+.Vt bio .
 .Pp
 This function should be used for every request which enters through
 the provider of a particular geom and needs to be scheduled down.
@@ -197,9 +240,12 @@
 .Fn g_duplicate_bio
 - same as
 .Fn g_clone_bio ,
-but always succeeds (allocates bio with the
+but always succeeds at the possible cost of blocking (allocates
+.Vt bio
+with the
 .Dv M_WAITOK
-malloc flag).
+.Xr uma_zalloc 9
+flag).
 .Pp
 The
 .Fn g_destroy_bio
@@ -256,6 +302,47 @@
 structure created by other means,
 .Fn g_reset_bio
 should be used to initialize it and between transactions.
+.Pp
+The
+.Fn g_io_new_uz
+function allocates a new
+.Xr uma 9
+zone for
+.Vt bio
+of
+.Va name
+as the zone name
+by
+.Xr uma_zcreate 9
+with the suitable argument.
+The new zone has no preallocated nor reserved items.
+The caller should prepare them by
+.Xr uma_prealloc 9
+and
+.Xr uma_zone_reserve 9
+before using it.
+The allocated
+.Xr uma 9
+zone can be freed by
+.Xr uma_zdestroy 9
+when it is no longer needed.
+.Pp
+The
+.Fn g_ctor_bio
+function is the
+.Xr uma 9
+zone item constructor for
+.Vt bio .
+Use this for the zones created without
+.Fn g_io_new_uz .
+The zones created by
+.Fn g_io_new_uz
+automatically have
+.Fn g_ctor_bio
+as the item constructor.
+There is no item destructor;
+.Vt bio
+does not depend on any external resources for now.
 .Sh RETURN VALUES
 The
 .Fn g_new_bio
@@ -266,6 +353,19 @@
 or
 .Dv NULL
 if an error occurred.
+.Pp
+The
+.Fn g_io_new_uz
+function returns a pointer to the allocated
+.Xr uma 9
+zone,
+or
+.Dv NULL
+if an error occurred.
+.Pp
+The
+.Fn g_ctor_bio
+function always returns zero to indicate a successful item construction.
 .Sh EXAMPLES
 Implementation of
 .Dq Dv NULL Ns -transformation ,
diff --git a/sys/geom/geom.h b/sys/geom/geom.h
--- a/sys/geom/geom.h
+++ b/sys/geom/geom.h
@@ -58,6 +58,7 @@
 struct g_configargs;
 struct disk_zone_args;
 struct thread;
+struct uma_zone;
 
 typedef int g_config_t (struct g_configargs *ca);
 typedef void g_ctl_req_t (struct gctl_req *, struct g_class *cp, char const *verb);
@@ -335,11 +336,15 @@
 int g_io_getattr(const char *attr, struct g_consumer *cp, int *len, void *ptr);
 int g_io_zonecmd(struct disk_zone_args *zone_args, struct g_consumer *cp);
 int g_io_flush(struct g_consumer *cp);
+struct uma_zone *g_io_new_uz(const char *name);
 int g_io_speedup(off_t shortage, u_int flags, size_t *resid,
     struct g_consumer *cp);
 void g_io_request(struct bio *bp, struct g_consumer *cp);
+int g_ctor_bio(void *mem, int size, void *arg, int flags);
 struct bio *g_new_bio(void);
+struct bio *g_new_bio_uz(struct uma_zone *uz);
 struct bio *g_alloc_bio(void);
+struct bio *g_alloc_bio_uz(struct uma_zone *uz);
 void g_reset_bio(struct bio *);
 void * g_read_data(struct g_consumer *cp, off_t offset, off_t length, int *error);
 int g_write_data(struct g_consumer *cp, off_t offset, void *ptr, off_t length);
diff --git a/sys/geom/geom_io.c b/sys/geom/geom_io.c
--- a/sys/geom/geom_io.c
+++ b/sys/geom/geom_io.c
@@ -137,17 +137,34 @@
 	return (bp);
 }
 
+static int __read_mostly reserved_new_bios = 65536;
+
 struct bio *
 g_new_bio(void)
+{
+	return (g_new_bio_uz(biozone));
+}
+
+struct bio *
+g_new_bio_uz(uma_zone_t uz)
 {
 	struct bio *bp;
 
-	bp = uma_zalloc(biozone, M_NOWAIT | M_ZERO);
+	if (NULL == uz)
+		uz = biozone;
+
+	bp = uma_zalloc_arg(uz, uz, M_NOWAIT |
+	    ((uz != biozone) || (reserved_new_bios > 0) ? M_USE_RESERVE : 0));
+	if (__predict_false(NULL == bp))
+		printf(
+		    "g_new_bio_uz(): failed to allocate from zone %s (sysctl: %s)\n",
+		    uma_zone_name(uz),
+		    uma_zone_unique_name(uz));
 #ifdef KTR
 	if (KTR_GEOM_ENABLED) {
 		struct stack st;
 
-		CTR1(KTR_GEOM, "g_new_bio(): %p", bp);
+		CTR1(KTR_GEOM, "g_new_bio_uz(): %p", bp);
 		stack_save(&st);
 		CTRSTACK(KTR_GEOM, &st, 3);
 	}
@@ -157,15 +174,24 @@
 
 struct bio *
 g_alloc_bio(void)
+{
+	return (g_alloc_bio_uz(biozone));
+}
+
+struct bio *
+g_alloc_bio_uz(struct uma_zone *uz)
 {
 	struct bio *bp;
 
-	bp = uma_zalloc(biozone, M_WAITOK | M_ZERO);
+	if (NULL == uz)
+		uz = biozone;
+
+	bp = uma_zalloc_arg(uz, uz, M_WAITOK);
 #ifdef KTR
 	if (KTR_GEOM_ENABLED) {
 		struct stack st;
 
-		CTR1(KTR_GEOM, "g_alloc_bio(): %p", bp);
+		CTR1(KTR_GEOM, "g_alloc_bio_uz(): %p", bp);
 		stack_save(&st);
 		CTRSTACK(KTR_GEOM, &st, 3);
 	}
@@ -176,6 +202,8 @@
 void
 g_destroy_bio(struct bio *bp)
 {
+	KASSERT(NULL != bp->bio_uz, ("null bio zone"));
+
 #ifdef KTR
 	if (KTR_GEOM_ENABLED) {
 		struct stack st;
@@ -185,7 +213,7 @@
 		CTRSTACK(KTR_GEOM, &st, 3);
 	}
 #endif
-	uma_zfree(biozone, bp);
+	uma_zfree(bp->bio_uz, bp);
 }
 
 struct bio *
@@ -193,8 +221,12 @@
 {
 	struct bio *bp2;
 
-	bp2 = uma_zalloc(biozone, M_NOWAIT | M_ZERO);
-	if (bp2 != NULL) {
+	KASSERT(NULL != bp->bio_uz, ("null bio zone"));
+
+	bp2 = uma_zalloc_arg(bp->bio_uz, bp->bio_uz, M_NOWAIT |
+	    ((bp->bio_uz != biozone) || (reserved_new_bios > 0) ?
+	    M_USE_RESERVE : 0));
+	if (__predict_true(bp2 != NULL)) {
 		bp2->bio_parent = bp;
 		bp2->bio_cmd = bp->bio_cmd;
 		/*
@@ -220,7 +252,11 @@
 		bp2->bio_track_bp = bp->bio_track_bp;
 #endif
 		bp->bio_children++;
-	}
+	} else
+		printf(
+		    "g_clone_bio(): failed to allocate from zone %s (sysctl: %s)\n",
+		    uma_zone_name(bp->bio_uz),
+		    uma_zone_unique_name(bp->bio_uz));
 #ifdef KTR
 	if (KTR_GEOM_ENABLED) {
 		struct stack st;
@@ -238,7 +274,9 @@
 {
 	struct bio *bp2;
 
-	bp2 = uma_zalloc(biozone, M_WAITOK | M_ZERO);
+	KASSERT(NULL != bp->bio_uz, ("null bio zone"));
+
+	bp2 = uma_zalloc_arg(bp->bio_uz, bp->bio_uz, M_WAITOK);
 	bp2->bio_flags = bp->bio_flags & (BIO_UNMAPPED | BIO_VLIST | BIO_SWAP);
 	bp2->bio_parent = bp;
 	bp2->bio_cmd = bp->bio_cmd;
@@ -265,8 +303,40 @@
 void
 g_reset_bio(struct bio *bp)
 {
+	uma_zone_t uz;
+
+	KASSERT(NULL != bp->bio_uz, ("null bio zone"));
 
+	uz = bp->bio_uz;
 	bzero(bp, sizeof(*bp));
+	bp->bio_uz = uz;
+}
+
+int
+g_ctor_bio(void *mem, int size __unused, void *arg, int flags __unused)
+{
+	struct bio *bp;
+	uma_zone_t uz;
+
+	bp = (struct bio *)mem;
+	uz = (uma_zone_t)arg;
+
+	KASSERT(sizeof(*bp) == size, ("bio size mismatch"));
+	KASSERT(NULL != uz, ("null bio zone"));
+
+	memset(bp, 0, sizeof(*bp));
+	bp->bio_uz = uz;
+
+	return (0);
+}
+
+uma_zone_t
+g_io_new_uz(const char *name)
+{
+	return (uma_zcreate(name, sizeof (struct bio),
+	    g_ctor_bio, NULL,
+	    NULL, NULL,
+	    0, 0));
 }
 
 void
@@ -275,10 +345,15 @@
 
 	g_bioq_init(&g_bio_run_down);
 	g_bioq_init(&g_bio_run_up);
-	biozone = uma_zcreate("g_bio", sizeof (struct bio),
-	    NULL, NULL,
-	    NULL, NULL,
-	    0, 0);
+	biozone = g_io_new_uz("g_bio");
+	/*
+	 * XXX the reservation of a uma(9) zone cannot be altered if it is serving
+	 * any items.
+	 */
+	if (reserved_new_bios > 0) {
+		uma_prealloc(biozone, reserved_new_bios);
+		uma_zone_reserve(biozone, reserved_new_bios);
+	}
 }
 
 int
@@ -734,6 +809,9 @@
 SYSCTL_INT(_kern_geom, OID_AUTO, inflight_transient_maps, CTLFLAG_RD,
     &inflight_transient_maps, 0,
     "Current count of the active transient maps");
+SYSCTL_INT(_kern_geom, OID_AUTO, reserved_new_bios, CTLFLAG_RDTUN,
+    &reserved_new_bios, 0,
+    "Number of reserved new bios for non-blocking allocation");
 
 static int
 g_io_transient_map_bio(struct bio *bp)
diff --git a/sys/sys/bio.h b/sys/sys/bio.h
--- a/sys/sys/bio.h
+++ b/sys/sys/bio.h
@@ -76,6 +76,11 @@
 struct disk;
 struct bio;
 struct vm_map;
+struct uma_zone;
+/*
+ * XXX The typedef of uma_zone_t here makes a typedef redefinition, even if
+ * the actual definition does not change.
+ */
 
 typedef void bio_task_t(void *);
 
@@ -113,6 +118,7 @@
 	u_int	bio_inbed;		/* Children safely home by now */
 	struct bio *bio_parent;		/* Pointer to parent */
 	struct bintime bio_t0;		/* Time request started */
+	struct uma_zone	*bio_uz;	/* uma(9) zone from which this bio comes */
 
 	bio_task_t *bio_task;		/* Task_queue handler */
 	void	*bio_task_arg;		/* Argument to above */