diff --git a/include/sys/ddt.h b/include/sys/ddt.h
index 671127154550..5f0d168bd45f 100644
--- a/include/sys/ddt.h
+++ b/include/sys/ddt.h
@@ -146,6 +146,7 @@ typedef struct {
 
 typedef struct {
 	ddt_phys_t	ddpf_phys[1];
+	uint64_t	ddpf_class_birth;
 } ddt_phys_flat_t;
 
 typedef struct {
@@ -162,6 +163,9 @@ typedef struct {
 #define	DDT_PHYS_FOR_COPIES(ddt, p)	_DDT_PHYS_SWITCH(ddt, 0, p)
 #define	DDT_PHYS_IS_DITTO(ddt, p)	_DDT_PHYS_SWITCH(ddt, 0, (p == 0))
 
+#define	DDT_PHYS_FLAT_CLASS_BIRTH(phys)	\
+	(((ddt_phys_flat_t *)(phys))->ddpf_class_birth)
+
 /*
  * A "live" entry, holding changes to an entry made this txg, and other data to
  * support loading, updating and repairing the entry.
diff --git a/include/sys/ddt_impl.h b/include/sys/ddt_impl.h
index e88a046ab8ae..d8365eb4a4f2 100644
--- a/include/sys/ddt_impl.h
+++ b/include/sys/ddt_impl.h
@@ -42,14 +42,17 @@ extern "C" {
 #define	DDT_DIR_FLAGS		"flags"
 
 /* Fill a lightweight entry from a live entry. */
-#define	DDT_ENTRY_TO_LIGHTWEIGHT(ddt, dde, ddlwe) do {		\
-	memset((ddlwe), 0, sizeof (*ddlwe));			\
-	(ddlwe)->ddlwe_key = (dde)->dde_key;			\
-	(ddlwe)->ddlwe_type = (dde)->dde_type;			\
-	(ddlwe)->ddlwe_class = (dde)->dde_class;		\
-	(ddlwe)->ddlwe_nphys = DDT_NPHYS(ddt);			\
-	for (int p = 0; p < (ddlwe)->ddlwe_nphys; p++)		\
-		(ddlwe)->ddlwe_phys[p] = (dde)->dde_phys[p];	\
+#define	DDT_ENTRY_TO_LIGHTWEIGHT(ddt, dde, ddlwe) do {			\
+	memset((ddlwe), 0, sizeof (*ddlwe));				\
+	(ddlwe)->ddlwe_key = (dde)->dde_key;				\
+	(ddlwe)->ddlwe_type = (dde)->dde_type;				\
+	(ddlwe)->ddlwe_class = (dde)->dde_class;			\
+	(ddlwe)->ddlwe_nphys = DDT_NPHYS(ddt);				\
+	for (int p = 0; p < (ddlwe)->ddlwe_nphys; p++)			\
+		(ddlwe)->ddlwe_phys[p] = (dde)->dde_phys[p];		\
+	if (ddt->ddt_flags & DDT_FLAG_FLAT)				\
+		DDT_PHYS_FLAT_CLASS_BIRTH(&((ddlwe)->ddlwe_phys[0])) =	\
+		DDT_PHYS_FLAT_CLASS_BIRTH(&((dde)->dde_phys[0]));	\
 } while (0)
 
 /*
diff --git a/module/zfs/ddt.c b/module/zfs/ddt.c
index 686b6f23ba24..c54add9ca364 100644
--- a/module/zfs/ddt.c
+++ b/module/zfs/ddt.c
@@ -849,6 +849,9 @@ ddt_lookup(ddt_t *ddt, const blkptr_t *bp, boolean_t add)
 
 	/* Time to make a new entry. */
 	dde = ddt_alloc(ddt, &search);
+	if (ddt->ddt_flags & DDT_FLAG_FLAT)
+		DDT_PHYS_FLAT_CLASS_BIRTH(&dde->dde_phys[0]) =
+		    gethrestime_sec();
 	avl_insert(&ddt->ddt_tree, dde, where);
 
 	/*