diff --git a/include/sys/ddt.h b/include/sys/ddt.h
index 7a09166909..222373c98a 100644
--- a/include/sys/ddt.h
+++ b/include/sys/ddt.h
@@ -151,16 +151,22 @@ typedef struct {
 #define	DDE_FLAG_LOADED		(1 << 0)	/* entry ready for use */
 #define	DDE_FLAG_OVERQUOTA	(1 << 1)	/* entry unusable, no space */
 
+/*
+ * Additional data to support entry update or repair. This is fixed size
+ * because its relatively rarely used.
+ */
 typedef struct {
-	/* key must be first for ddt_key_compare */
-	ddt_key_t	dde_key;		/* ddt_tree key */
-	ddt_phys_t	dde_phys[DDT_PHYS_MAX];	/* on-disk data */
+	/* copy of data after a repair read, to be rewritten */
+	abd_t		*dde_repair_abd;
 
 	/* in-flight update IOs */
 	zio_t		*dde_lead_zio[DDT_PHYS_MAX];
+} ddt_entry_io_t;
 
-	/* copy of data after a repair read, to be rewritten */
-	struct abd	*dde_repair_abd;
+typedef struct {
+	/* key must be first for ddt_key_compare */
+	ddt_key_t	dde_key;	/* ddt_tree key */
+	avl_node_t	dde_node;	/* ddt_tree_node */
 
 	/* storage type and class the entry was loaded from */
 	ddt_type_t	dde_type;
@@ -170,7 +176,9 @@ typedef struct {
 	kcondvar_t	dde_cv;		/* signaled when load completes */
 	uint64_t	dde_waiters;	/* count of waiters on dde_cv */
 
-	avl_node_t	dde_node;	/* ddt_tree node */
+	ddt_entry_io_t	*dde_io;	/* IO support, when required */
+
+	ddt_phys_t	dde_phys[];	/* physical data */
 } ddt_entry_t;
 
 /*
@@ -265,6 +273,8 @@ extern void ddt_prefetch_all(spa_t *spa);
 extern boolean_t ddt_class_contains(spa_t *spa, ddt_class_t max_class,
     const blkptr_t *bp);
 
+extern void ddt_alloc_entry_io(ddt_entry_t *dde);
+
 extern ddt_entry_t *ddt_repair_start(ddt_t *ddt, const blkptr_t *bp);
 extern void ddt_repair_done(ddt_t *ddt, ddt_entry_t *dde);
 
diff --git a/module/zfs/ddt.c b/module/zfs/ddt.c
index aac2250bf3..213e042394 100644
--- a/module/zfs/ddt.c
+++ b/module/zfs/ddt.c
@@ -164,6 +164,9 @@
 static kmem_cache_t *ddt_cache;
 static kmem_cache_t *ddt_entry_cache;
 
+#define	DDT_ENTRY_SIZE	\
+	(sizeof (ddt_entry_t) + sizeof (ddt_phys_t) * DDT_PHYS_MAX)
+
 /*
  * Enable/disable prefetching of dedup-ed blocks which are going to be freed.
  */
@@ -343,7 +346,7 @@ ddt_object_lookup(ddt_t *ddt, ddt_type_t type, ddt_class_t class,
 
 	return (ddt_ops[type]->ddt_op_lookup(ddt->ddt_os,
 	    ddt->ddt_object[type][class], &dde->dde_key,
-	    dde->dde_phys, sizeof (dde->dde_phys)));
+	    dde->dde_phys, sizeof (ddt_phys_t) * DDT_NPHYS(ddt)));
 }
 
 static int
@@ -386,7 +389,7 @@ ddt_object_update(ddt_t *ddt, ddt_type_t type, ddt_class_t class,
 
 	return (ddt_ops[type]->ddt_op_update(ddt->ddt_os,
 	    ddt->ddt_object[type][class], &dde->dde_key, dde->dde_phys,
-	    sizeof (dde->dde_phys), tx));
+	    sizeof (ddt_phys_t) * DDT_NPHYS(ddt), tx));
 }
 
 static int
@@ -597,7 +600,7 @@ ddt_init(void)
 	ddt_cache = kmem_cache_create("ddt_cache",
 	    sizeof (ddt_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
 	ddt_entry_cache = kmem_cache_create("ddt_entry_cache",
-	    sizeof (ddt_entry_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
+	    DDT_ENTRY_SIZE, 0, NULL, NULL, NULL, NULL, NULL, 0);
 }
 
 void
@@ -613,7 +616,7 @@ ddt_alloc(const ddt_key_t *ddk)
 	ddt_entry_t *dde;
 
 	dde = kmem_cache_alloc(ddt_entry_cache, KM_SLEEP);
-	memset(dde, 0, sizeof (ddt_entry_t));
+	memset(dde, 0, DDT_ENTRY_SIZE);
 	cv_init(&dde->dde_cv, NULL, CV_DEFAULT, NULL);
 
 	dde->dde_key = *ddk;
@@ -621,14 +624,27 @@ ddt_alloc(const ddt_key_t *ddk)
 	return (dde);
 }
 
+void
+ddt_alloc_entry_io(ddt_entry_t *dde)
+{
+	if (dde->dde_io != NULL)
+		return;
+
+	dde->dde_io = kmem_zalloc(sizeof (ddt_entry_io_t), KM_SLEEP);
+}
+
 static void
 ddt_free(const ddt_t *ddt, ddt_entry_t *dde)
 {
-	for (int p = 0; p < DDT_NPHYS(ddt); p++)
-		ASSERT3P(dde->dde_lead_zio[p], ==, NULL);
+	if (dde->dde_io != NULL) {
+		for (int p = 0; p < DDT_NPHYS(ddt); p++)
+			ASSERT3P(dde->dde_io->dde_lead_zio[p], ==, NULL);
 
-	if (dde->dde_repair_abd != NULL)
-		abd_free(dde->dde_repair_abd);
+		if (dde->dde_io->dde_repair_abd != NULL)
+			abd_free(dde->dde_io->dde_repair_abd);
+
+		kmem_free(dde->dde_io, sizeof (ddt_entry_io_t));
+	}
 
 	cv_destroy(&dde->dde_cv);
 	kmem_cache_free(ddt_entry_cache, dde);
@@ -1191,6 +1207,7 @@ ddt_repair_start(ddt_t *ddt, const blkptr_t *bp)
 	ddt_key_fill(&ddk, bp);
 
 	dde = ddt_alloc(&ddk);
+	ddt_alloc_entry_io(dde);
 
 	for (ddt_type_t type = 0; type < DDT_TYPES; type++) {
 		for (ddt_class_t class = 0; class < DDT_CLASSES; class++) {
@@ -1205,7 +1222,7 @@ ddt_repair_start(ddt_t *ddt, const blkptr_t *bp)
 		}
 	}
 
-	memset(dde->dde_phys, 0, sizeof (dde->dde_phys));
+	memset(dde->dde_phys, 0, sizeof (ddt_phys_t) * DDT_NPHYS(ddt));
 
 	return (dde);
 }
@@ -1217,7 +1234,8 @@ ddt_repair_done(ddt_t *ddt, ddt_entry_t *dde)
 
 	ddt_enter(ddt);
 
-	if (dde->dde_repair_abd != NULL && spa_writeable(ddt->ddt_spa) &&
+	if (dde->dde_io->dde_repair_abd != NULL &&
+	    spa_writeable(ddt->ddt_spa) &&
 	    avl_find(&ddt->ddt_repair_tree, dde, &where) == NULL)
 		avl_insert(&ddt->ddt_repair_tree, dde, where);
 	else
@@ -1255,8 +1273,9 @@ ddt_repair_entry(ddt_t *ddt, ddt_entry_t *dde, ddt_entry_t *rdde, zio_t *rio)
 			continue;
 		ddt_bp_create(ddt->ddt_checksum, ddk, ddp, &blk);
 		zio_nowait(zio_rewrite(zio, zio->io_spa, 0, &blk,
-		    rdde->dde_repair_abd, DDK_GET_PSIZE(rddk), NULL, NULL,
-		    ZIO_PRIORITY_SYNC_WRITE, ZIO_DDT_CHILD_FLAGS(zio), NULL));
+		    rdde->dde_io->dde_repair_abd, DDK_GET_PSIZE(rddk),
+		    NULL, NULL, ZIO_PRIORITY_SYNC_WRITE,
+		    ZIO_DDT_CHILD_FLAGS(zio), NULL));
 	}
 
 	zio_nowait(zio);
@@ -1301,7 +1320,8 @@ ddt_sync_entry(ddt_t *ddt, ddt_entry_t *dde, dmu_tx_t *tx, uint64_t txg)
 	ASSERT(dde->dde_flags & DDE_FLAG_LOADED);
 
 	for (int p = 0; p < DDT_NPHYS(ddt); p++) {
-		ASSERT3P(dde->dde_lead_zio[p], ==, NULL);
+		ASSERT(dde->dde_io == NULL ||
+		    dde->dde_io->dde_lead_zio[p] == NULL);
 		ddt_phys_t *ddp = &dde->dde_phys[p];
 		if (ddp->ddp_phys_birth == 0) {
 			ASSERT0(ddp->ddp_refcnt);
diff --git a/module/zfs/zio.c b/module/zfs/zio.c
index 914f83fb9f..1ca71c738c 100644
--- a/module/zfs/zio.c
+++ b/module/zfs/zio.c
@@ -3265,8 +3265,8 @@ zio_ddt_child_read_done(zio_t *zio)
 	if (zio->io_error == 0)
 		ddt_phys_clear(ddp);	/* this ddp doesn't need repair */
 
-	if (zio->io_error == 0 && dde->dde_repair_abd == NULL)
-		dde->dde_repair_abd = zio->io_abd;
+	if (zio->io_error == 0 && dde->dde_io->dde_repair_abd == NULL)
+		dde->dde_io->dde_repair_abd = zio->io_abd;
 	else
 		abd_free(zio->io_abd);
 	mutex_exit(&pio->io_lock);
@@ -3340,8 +3340,8 @@ zio_ddt_read_done(zio_t *zio)
 			zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, B_FALSE);
 			return (NULL);
 		}
-		if (dde->dde_repair_abd != NULL) {
-			abd_copy(zio->io_abd, dde->dde_repair_abd,
+		if (dde->dde_io->dde_repair_abd != NULL) {
+			abd_copy(zio->io_abd, dde->dde_io->dde_repair_abd,
 			    zio->io_size);
 			zio->io_child_error[ZIO_CHILD_DDT] = 0;
 		}
@@ -3378,7 +3378,7 @@ zio_ddt_collision(zio_t *zio, ddt_t *ddt, ddt_entry_t *dde)
 		if (DDT_PHYS_IS_DITTO(ddt, p))
 			continue;
 
-		zio_t *lio = dde->dde_lead_zio[p];
+		zio_t *lio = dde->dde_io->dde_lead_zio[p];
 
 		if (lio != NULL && do_raw) {
 			return (lio->io_size != zio->io_size ||
@@ -3472,7 +3472,7 @@ zio_ddt_child_write_ready(zio_t *zio)
 
 	ddt_enter(ddt);
 
-	ASSERT(dde->dde_lead_zio[p] == zio);
+	ASSERT(dde->dde_io->dde_lead_zio[p] == zio);
 
 	ddt_phys_fill(ddp, zio->io_bp);
 
@@ -3495,8 +3495,8 @@ zio_ddt_child_write_done(zio_t *zio)
 	ddt_enter(ddt);
 
 	ASSERT(ddp->ddp_refcnt == 0);
-	ASSERT(dde->dde_lead_zio[p] == zio);
-	dde->dde_lead_zio[p] = NULL;
+	ASSERT(dde->dde_io->dde_lead_zio[p] == zio);
+	dde->dde_io->dde_lead_zio[p] = NULL;
 
 	if (zio->io_error == 0) {
 		zio_link_t *zl = NULL;
@@ -3563,11 +3563,13 @@ zio_ddt_write(zio_t *zio)
 		return (zio);
 	}
 
-	if (ddp->ddp_phys_birth != 0 || dde->dde_lead_zio[p] != NULL) {
+	ddt_alloc_entry_io(dde);
+
+	if (ddp->ddp_phys_birth != 0 || dde->dde_io->dde_lead_zio[p] != NULL) {
 		if (ddp->ddp_phys_birth != 0)
 			ddt_bp_fill(ddp, bp, txg);
-		if (dde->dde_lead_zio[p] != NULL)
-			zio_add_child(zio, dde->dde_lead_zio[p]);
+		if (dde->dde_io->dde_lead_zio[p] != NULL)
+			zio_add_child(zio, dde->dde_io->dde_lead_zio[p]);
 		else
 			ddt_phys_addref(ddp);
 	} else if (zio->io_bp_override) {
@@ -3583,7 +3585,7 @@ zio_ddt_write(zio_t *zio)
 		    ZIO_DDT_CHILD_FLAGS(zio), &zio->io_bookmark);
 
 		zio_push_transform(cio, zio->io_abd, zio->io_size, 0, NULL);
-		dde->dde_lead_zio[p] = cio;
+		dde->dde_io->dde_lead_zio[p] = cio;
 	}
 
 	ddt_exit(ddt);