diff --git a/include/sys/Makefile.am b/include/sys/Makefile.am
index 73c4a84217..a16bd6ce6f 100644
--- a/include/sys/Makefile.am
+++ b/include/sys/Makefile.am
@@ -29,6 +29,7 @@ KERNEL_H = \
 	$(top_srcdir)/include/sys/dirent.h \
 	$(top_srcdir)/include/sys/disp.h \
 	$(top_srcdir)/include/sys/dkio.h \
+	$(top_srcdir)/include/sys/dkioc_free_util.h \
 	$(top_srcdir)/include/sys/dklabel.h \
 	$(top_srcdir)/include/sys/dnlc.h \
 	$(top_srcdir)/include/sys/dumphdr.h \
diff --git a/include/sys/dkio.h b/include/sys/dkio.h
index d8c700718f..dd7a95f137 100644
--- a/include/sys/dkio.h
+++ b/include/sys/dkio.h
@@ -25,14 +25,16 @@
 #ifndef _SPL_DKIO_H
 #define	_SPL_DKIO_H
 
-struct dk_callback {
-	void (*dkc_callback)(void *dkc_cookie, int error);
-	void *dkc_cookie;
-	int dkc_flag;
-};
+#define	DFL_SZ(num_exts) \
+	(sizeof (dkioc_free_list_t) + (num_exts - 1) * 16)
 
-#define	DKIOC			(0x04 << 8)
-#define	DKIOCFLUSHWRITECACHE	(DKIOC | 34)
-#define	DKIOCTRIM		(DKIOC | 35)
+#define	DKIOC		(0x04 << 8)
+#define	DKIOCFLUSHWRITECACHE	(DKIOC|34)	/* flush cache to phys medium */
+
+/*
+ * ioctl to free space (e.g. SCSI UNMAP) off a disk.
+ * Pass a dkioc_free_list_t containing a list of extents to be freed.
+ */
+#define	DKIOCFREE	(DKIOC|50)
 
 #endif /* _SPL_DKIO_H */
diff --git a/include/sys/dkioc_free_util.h b/include/sys/dkioc_free_util.h
new file mode 100644
index 0000000000..a424d42320
--- /dev/null
+++ b/include/sys/dkioc_free_util.h
@@ -0,0 +1,54 @@
+/*****************************************************************************\
+ *  Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
+ *  Copyright (C) 2007 The Regents of the University of California.
+ *  Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
+ *  Written by Brian Behlendorf <behlendorf1@llnl.gov>.
+ *  UCRL-CODE-235197
+ *
+ *  This file is part of the SPL, Solaris Porting Layer.
+ *  For details, see <http://zfsonlinux.org/>.
+ *
+ *  The SPL is free software; you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License as published by the
+ *  Free Software Foundation; either version 2 of the License, or (at your
+ *  option) any later version.
+ *
+ *  The SPL is distributed in the hope that it will be useful, but WITHOUT
+ *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ *  for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
+\*****************************************************************************/
+
+#ifndef _SPL_DKIOC_UTIL_H
+#define	_SPL_DKIOC_UTIL_H
+
+#include <sys/dkio.h>
+
+typedef struct dkioc_free_list_ext_s {
+	uint64_t		dfle_start;
+	uint64_t		dfle_length;
+} dkioc_free_list_ext_t;
+
+typedef struct dkioc_free_list_s {
+	uint64_t		dfl_flags;
+	uint64_t		dfl_num_exts;
+	int64_t			dfl_offset;
+
+	/*
+	 * N.B. this is only an internal debugging API! This is only called
+	 * from debug builds of sd for pre-release checking. Remove before GA!
+	 */
+	void			(*dfl_ck_func)(uint64_t, uint64_t, void *);
+	void			*dfl_ck_arg;
+
+	dkioc_free_list_ext_t	dfl_exts[1];
+} dkioc_free_list_t;
+
+static inline void dfl_free(dkioc_free_list_t *dfl) {
+	kmem_free(dfl, DFL_SZ(dfl->dfl_num_exts));
+}
+
+#endif /* _SPL_DKIOC_UTIL_H */