diff --git a/sbin/fsck_ffs/fsck.h b/sbin/fsck_ffs/fsck.h
index 342b95f138e6..250fc1f9890f 100644
--- a/sbin/fsck_ffs/fsck.h
+++ b/sbin/fsck_ffs/fsck.h
@@ -274,6 +274,7 @@ char	yflag;			/* assume a yes response */
 int	bkgrdflag;		/* use a snapshot to run on an active system */
 int	bflag;			/* location of alternate super block */
 int	debug;			/* output debugging info */
+int	Eflag;			/* zero out empty data blocks */
 int	inoopt;			/* trim out unused inodes */
 char	ckclean;		/* only do work if not cleanly unmounted */
 int	cvtlevel;		/* convert to newer file system format */
@@ -337,6 +338,7 @@ char	       *blockcheck(char *name);
 int		blread(int fd, char *buf, ufs2_daddr_t blk, long size);
 void		bufinit(void);
 void		blwrite(int fd, char *buf, ufs2_daddr_t blk, long size);
+void		blerase(int fd, ufs2_daddr_t blk, long size);
 void		cacheino(union dinode *dp, ino_t inumber);
 void		catch(int);
 void		catchquit(int);
diff --git a/sbin/fsck_ffs/fsck_ffs.8 b/sbin/fsck_ffs/fsck_ffs.8
index 2834149f97c8..523cb6543ed8 100644
--- a/sbin/fsck_ffs/fsck_ffs.8
+++ b/sbin/fsck_ffs/fsck_ffs.8
@@ -29,7 +29,7 @@
 .\"	@(#)fsck.8	8.4 (Berkeley) 5/9/95
 .\" $FreeBSD$
 .\"
-.Dd January 25, 2009
+.Dd April 27, 2011
 .Dt FSCK_FFS 8
 .Os
 .Sh NAME
@@ -38,7 +38,7 @@
 .Nd file system consistency check and interactive repair
 .Sh SYNOPSIS
 .Nm
-.Op Fl BFfnpry
+.Op Fl BEFfnpry
 .Op Fl b Ar block
 .Op Fl c Ar level
 .Op Fl m Ar mode
@@ -149,6 +149,24 @@ If unexpected errors are found,
 the file system is marked as needing a foreground check and
 .Nm
 exits without attempting any further cleaning.
+.It Fl E
+Clear unallocated blocks, notifying the underlying device that they
+are not used and that their contents may be discarded.
+This is useful for filesystems which have been mounted on systems
+without TRIM support, or with TRIM support disabled, as well as
+filesystems which have been copied from one device to another.
+.Pp
+See the
+.Fl E
+and
+.Fl t
+flags of
+.Xr newfs 8 ,
+and
+the
+.Fl t
+flag of
+.Xr tunefs 8 .
 .It Fl F
 Determine whether the file system needs to be cleaned immediately
 in foreground, or if its cleaning can be deferred to background.
diff --git a/sbin/fsck_ffs/fsutil.c b/sbin/fsck_ffs/fsutil.c
index 68d113a3f618..85ea03352685 100644
--- a/sbin/fsck_ffs/fsutil.c
+++ b/sbin/fsck_ffs/fsutil.c
@@ -39,9 +39,10 @@ __FBSDID("$FreeBSD$");
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/sysctl.h>
+#include <sys/disk.h>
 #include <sys/disklabel.h>
+#include <sys/ioctl.h>
 #include <sys/stat.h>
-#include <sys/disklabel.h>
 
 #include <ufs/ufs/dinode.h>
 #include <ufs/ufs/dir.h>
@@ -421,6 +422,20 @@ blwrite(int fd, char *buf, ufs2_daddr_t blk, long size)
 	return;
 }
 
+void
+blerase(int fd, ufs2_daddr_t blk, long size)
+{
+	off_t ioarg[2];
+
+	if (fd < 0)
+		return;
+	ioarg[0] = blk * dev_bsize;
+	ioarg[1] = size;
+	ioctl(fd, DIOCGDELETE, ioarg);
+	/* we don't really care if we succeed or not */
+	return;
+}
+
 /*
  * Verify cylinder group's magic number and other parameters.  If the
  * test fails, offer an option to rebuild the whole cylinder group.
diff --git a/sbin/fsck_ffs/main.c b/sbin/fsck_ffs/main.c
index 127a612cae91..e26494d50574 100644
--- a/sbin/fsck_ffs/main.c
+++ b/sbin/fsck_ffs/main.c
@@ -82,7 +82,7 @@ main(int argc, char *argv[])
 	sync();
 	skipclean = 1;
 	inoopt = 0;
-	while ((ch = getopt(argc, argv, "b:Bc:CdfFm:npry")) != -1) {
+	while ((ch = getopt(argc, argv, "b:Bc:CdEfFm:npry")) != -1) {
 		switch (ch) {
 		case 'b':
 			skipclean = 0;
@@ -106,6 +106,10 @@ main(int argc, char *argv[])
 			debug++;
 			break;
 
+		case 'E':
+			Eflag++;
+			break;
+
 		case 'f':
 			skipclean = 0;
 			break;
@@ -632,7 +636,7 @@ static void
 usage(void)
 {
 	(void) fprintf(stderr,
-	    "usage: %s [-BFprfny] [-b block] [-c level] [-m mode] "
+	    "usage: %s [-BEFprfny] [-b block] [-c level] [-m mode] "
 			"filesystem ...\n",
 	    getprogname());
 	exit(1);
diff --git a/sbin/fsck_ffs/pass5.c b/sbin/fsck_ffs/pass5.c
index 639ce0f8abdc..01ed8a54052d 100644
--- a/sbin/fsck_ffs/pass5.c
+++ b/sbin/fsck_ffs/pass5.c
@@ -51,6 +51,8 @@ __FBSDID("$FreeBSD$");
 
 static void check_maps(u_char *, u_char *, int, ufs2_daddr_t, const char *, int *, int, int);
 
+static void clear_blocks(ufs2_daddr_t start, ufs2_daddr_t end);
+
 void
 pass5(void)
 {
@@ -58,7 +60,7 @@ pass5(void)
 	int inomapsize, blkmapsize;
 	struct fs *fs = &sblock;
 	struct cg *cg = &cgrp;
-	ufs2_daddr_t d, dbase, dmax;
+	ufs2_daddr_t d, dbase, dmax, start;
 	int excessdirs, rewritecg = 0;
 	struct csum *cs;
 	struct csum_total cstotal;
@@ -242,13 +244,21 @@ pass5(void)
 				setbit(cg_inosused(newcg), i);
 				newcg->cg_cs.cs_nifree--;
 			}
+		start = -1;
 		for (i = 0, d = dbase;
 		     d < dmax;
 		     d += fs->fs_frag, i += fs->fs_frag) {
 			frags = 0;
 			for (j = 0; j < fs->fs_frag; j++) {
-				if (testbmap(d + j))
+				if (testbmap(d + j)) {
+					if (Eflag && start != -1) {
+						clear_blocks(start, d + j - 1);
+						start = -1;
+					}
 					continue;
+				}
+				if (start == -1)
+					start = d + j;
 				setbit(cg_blksfree(newcg), i + j);
 				frags++;
 			}
@@ -263,6 +273,8 @@ pass5(void)
 				ffs_fragacct(fs, blk, newcg->cg_frsum, 1);
 			}
 		}
+		if (Eflag && start != -1)
+			clear_blocks(start, d - 1);
 		if (fs->fs_contigsumsize > 0) {
 			int32_t *sump = cg_clustersum(newcg);
 			u_char *mapp = cg_clustersfree(newcg);
@@ -551,3 +563,12 @@ check_maps(
 		}
 	}
 }
+
+static void clear_blocks(ufs2_daddr_t start, ufs2_daddr_t end)
+{
+
+	if (debug)
+		printf("Zero frags %jd to %jd\n", start, end);
+	blerase(fswritefd, fsbtodb(&sblock, start),
+	    lfragtosize(&sblock, end - start + 1));
+}