Index: sys/geom/geom_dev.c
===================================================================
--- sys/geom/geom_dev.c
+++ sys/geom/geom_dev.c
@@ -111,19 +111,24 @@
     "delete request sent to the provider. Larger requests are chunked "
     "so they can be interrupted. (0 = disable chunking)");
 
+static char *env_dumpdev = NULL;
 static char *dumpdev = NULL;
 static void
 g_dev_init(struct g_class *mp)
 {
+	static const char *devprefix = "/dev/";
 
-	dumpdev = kern_getenv("dumpdev");
+	dumpdev = env_dumpdev = kern_getenv("dumpdev");
+	if (strncmp(dumpdev, devprefix, strlen(devprefix)) == 0)
+		dumpdev += strlen(devprefix);
 }
 
 static void
 g_dev_fini(struct g_class *mp)
 {
 
-	freeenv(dumpdev);
+	freeenv(env_dumpdev);
+	dumpdev = env_dumpdev = NULL;
 }
 
 static int
@@ -158,8 +163,8 @@
 	if (strcmp(devtoname(dev), dumpdev) != 0)
 		return;
 	if (g_dev_setdumpdev(dev, curthread) == 0) {
-		freeenv(dumpdev);
-		dumpdev = NULL;
+		freeenv(env_dumpdev);
+		dumpdev = env_dumpdev = NULL;
 	}
 }