diff --git a/sbin/growfs/growfs.c b/sbin/growfs/growfs.c
index 5e950fccd112..bce4d59adc1c 100644
--- a/sbin/growfs/growfs.c
+++ b/sbin/growfs/growfs.c
@@ -371,16 +371,16 @@ static void
 initcg(int cylno, time_t utime, int fso, unsigned int Nflag)
 {
 	DBG_FUNC("initcg")
-	static void *iobuf;
+	static caddr_t iobuf;
 	long blkno, start;
 	ufs2_daddr_t i, cbase, dmax;
 	struct ufs1_dinode *dp1;
 	struct csum *cs;
 	uint d, dupper, dlower;
 
-	if (iobuf == NULL && (iobuf = malloc(sblock.fs_bsize)) == NULL) {
+	if (iobuf == NULL && (iobuf = malloc(sblock.fs_bsize * 3)) == NULL)
 		errx(37, "panic: cannot allocate I/O buffer");
-	}
+
 	/*
 	 * Determine block bounds for cylinder group.
 	 * Allow space for super block summary information in first
@@ -396,17 +396,12 @@ initcg(int cylno, time_t utime, int fso, unsigned int Nflag)
 		dupper += howmany(sblock.fs_cssize, sblock.fs_fsize);
 	cs = &fscs[cylno];
 	memset(&acg, 0, sblock.fs_cgsize);
-	/*
-	 * Note that we do not set cg_initediblk at all.
-	 * In this extension of a previous filesystem
-	 * we have no inodes initialized for the cylinder
-	 * group at all. The first access to that cylinder
-	 * group will do the correct initialization.
-	 */
 	acg.cg_time = utime;
 	acg.cg_magic = CG_MAGIC;
 	acg.cg_cgx = cylno;
 	acg.cg_niblk = sblock.fs_ipg;
+	acg.cg_initediblk = sblock.fs_ipg < 2 * INOPB(&sblock) ?
+	    sblock.fs_ipg : 2 * INOPB(&sblock);
 	acg.cg_ndblk = dmax - cbase;
 	if (sblock.fs_contigsumsize > 0)
 		acg.cg_nclusterblks = acg.cg_ndblk / sblock.fs_frag;
@@ -419,6 +414,7 @@ initcg(int cylno, time_t utime, int fso, unsigned int Nflag)
 		acg.cg_time = 0;
 		acg.cg_old_niblk = acg.cg_niblk;
 		acg.cg_niblk = 0;
+		acg.cg_initediblk = 0;
 		acg.cg_old_btotoff = start;
 		acg.cg_old_boff = acg.cg_old_btotoff +
 		    sblock.fs_old_cpg * sizeof(int32_t);
@@ -538,11 +534,14 @@ initcg(int cylno, time_t utime, int fso, unsigned int Nflag)
 	sblock.fs_cstotal.cs_nbfree += acg.cg_cs.cs_nbfree;
 	sblock.fs_cstotal.cs_nifree += acg.cg_cs.cs_nifree;
 	*cs = acg.cg_cs;
+
+	memcpy(iobuf, &acg, sblock.fs_cgsize);
+	memset(iobuf + sblock.fs_cgsize, '\0',
+	    sblock.fs_bsize * 3 - sblock.fs_cgsize);
+
 	wtfs(fsbtodb(&sblock, cgtod(&sblock, cylno)),
-		sblock.fs_bsize, (char *)&acg, fso, Nflag);
-	DBG_DUMP_CG(&sblock,
-	    "new cg",
-	    &acg);
+	    sblock.fs_bsize * 3, iobuf, fso, Nflag);
+	DBG_DUMP_CG(&sblock, "new cg", &acg);
 
 	DBG_LEAVE;
 	return;
diff --git a/tools/regression/sbin/Makefile b/tools/regression/sbin/Makefile
new file mode 100644
index 000000000000..33d158e7d022
--- /dev/null
+++ b/tools/regression/sbin/Makefile
@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+SUBDIR=	growfs
+
+.include <bsd.subdir.mk>
diff --git a/tools/regression/sbin/growfs/Makefile b/tools/regression/sbin/growfs/Makefile
new file mode 100644
index 000000000000..dc9fa6771f24
--- /dev/null
+++ b/tools/regression/sbin/growfs/Makefile
@@ -0,0 +1,6 @@
+# $FreeBSD$
+
+all test:
+	prove -vmw regress.t
+
+clean:
diff --git a/tools/regression/sbin/growfs/regress.t b/tools/regression/sbin/growfs/regress.t
new file mode 100755
index 000000000000..9dbdd8523970
--- /dev/null
+++ b/tools/regression/sbin/growfs/regress.t
@@ -0,0 +1,91 @@
+#! /usr/bin/perl
+#
+# $FreeBSD$
+
+use strict;
+use warnings;
+use Test::More tests => 19;
+use Fcntl qw(:DEFAULT :seek);
+
+use constant BLK => 512;
+use constant BLKS_PER_MB => 2048;
+
+my $unit;
+END { system "mdconfig -du$unit" if defined $unit };
+
+sub setsize {
+    my ($partszMB, $unitszMB) = @_;
+
+    open my $fd, "|-", "disklabel -R md$unit /dev/stdin" or die;
+    print $fd "a: ", ($partszMB * BLKS_PER_MB), " 0 4.2BSD 1024 8192\n";
+    print $fd "c: ", ($unitszMB * BLKS_PER_MB), " 0 unused 0 0\n";
+    close $fd;
+}
+
+sub fill {
+    my ($start, $size, $content) = @_;
+
+    my $content512 = $content x (int(512 / length $content) + 1);
+    substr($content512, 512) = "";
+    sysopen my $fd, "/dev/md$unit", O_RDWR or die "/dev/md$unit: $!";
+    seek($fd, $start * BLK, SEEK_SET);
+    while ($size) {
+	syswrite($fd, $content512) == 512 or die "write: $!";
+	$size--;
+    }
+}
+
+SKIP: {
+    skip "Cannot test without UID 0", 19 if $<;
+
+    chomp(my $md = `mdconfig -s40m`);
+    like($md, qr/^md\d+$/, "Created $md with size 40m") or die;
+    $unit = substr $md, 2;
+
+    for my $type (1..2) {
+
+	initialise: {
+	    ok(setsize(10, 40), "Sized ${md}a to 10m");
+	    system "newfs -O $type -U ${md}a >/dev/null";
+	    is($?, 0, "Initialised the filesystem on ${md}a as UFS$type");
+	    chomp(my @out = `fsck -tufs -y ${md}a`);
+	    ok(!grep(/MODIFIED/, @out), "fsck says ${md}a is clean, " .
+		scalar(@out) . " lines of output");
+	}
+
+	extend20_zeroed: {
+	    ok(setsize(20, 40), "Sized ${md}a to 20m");
+	    diag "Filling the extent with zeros";
+	    fill(10 * BLKS_PER_MB, 10 * BLKS_PER_MB, chr(0));
+	    my $out = `growfs -y ${md}a`;
+	    is($?, 0, "Extended the filesystem on ${md}a") or print $out;
+
+	    my ($unallocated) = $out =~ m{\d+ sectors cannot be allocated};
+	    fill(30 * BLKS_PER_MB - $unallocated, $unallocated, chr(0))
+		if $unallocated;
+
+	    chomp(my @out = `fsck -tufs -y ${md}a`);
+	    ok(!grep(/MODIFIED/, @out), "fsck says ${md}a is clean, " .
+		scalar(@out) . " lines of output");
+	}
+
+	extend30_garbaged: {
+	    ok(setsize(30, 40), "Sized ${md}a to 30m");
+	    diag "Filling the extent with garbage";
+	    fill(20 * BLKS_PER_MB, 10 * BLKS_PER_MB, chr(0xaa) . chr(0x55));
+	    my $out = `growfs -y ${md}a`;
+	    is($?, 0, "Extended the filesystem on ${md}a") or print $out;
+
+	    my ($unallocated) = $out =~ m{\d+ sectors cannot be allocated};
+	    fill(30 * BLKS_PER_MB - $unallocated, $unallocated, chr(0))
+		if $unallocated;
+
+	    chomp(my @out = `fsck -tufs -y ${md}a`);
+	    ok(!grep(/MODIFIED/, @out), "fsck says ${md}a is clean, " .
+		scalar(@out) . " lines of output");
+	}
+    }
+
+    system "mdconfig -du$unit";
+    undef $unit;
+}