diff --git a/include/libxfs.h b/include/libxfs.h
index 2b24439..81ee208 100644
--- a/include/libxfs.h
+++ b/include/libxfs.h
@@ -182,6 +182,8 @@
 
 extern int	libxfs_alloc_file_space(struct xfs_inode *ip, xfs_off_t offset,
 					xfs_off_t len, uint32_t bmapi_flags);
+extern int	libxfs_file_write(struct xfs_trans *tp, struct xfs_inode *ip,
+				  void *buf, size_t len, bool logit);
 
 /* XXX: this is messy and needs fixing */
 #ifndef __LIBXFS_INTERNAL_XFS_H__
diff --git a/libxfs/util.c b/libxfs/util.c
index 05a036e..6715a34 100644
--- a/libxfs/util.c
+++ b/libxfs/util.c
@@ -533,3 +533,72 @@
 	return ret;
 }
 #endif
+
+/*
+ * Write a buffer to a file on the data device.  We assume there are no holes
+ * and no unwritten extents.
+ */
+int
+libxfs_file_write(
+	struct xfs_trans	*tp,
+	struct xfs_inode	*ip,
+	void			*buf,
+	size_t			len,
+	bool			logit)
+{
+	struct xfs_bmbt_irec	map;
+	struct xfs_mount	*mp = ip->i_mount;
+	struct xfs_buf		*bp;
+	xfs_fileoff_t		bno = 0;
+	xfs_fileoff_t		end_bno = XFS_B_TO_FSB(mp, len);
+	size_t			count;
+	size_t			bcount;
+	int			nmap;
+	int			error = 0;
+
+	/* Write up to 1MB at a time. */
+	while (bno < end_bno) {
+		xfs_filblks_t	maplen;
+
+		maplen = min(end_bno - bno, XFS_B_TO_FSBT(mp, 1048576));
+		nmap = 1;
+		error = libxfs_bmapi_read(ip, bno, maplen, &map, &nmap, 0);
+		if (error)
+			return error;
+		if (nmap != 1)
+			return -ENOSPC;
+
+		if (map.br_startblock == HOLESTARTBLOCK ||
+		    map.br_state == XFS_EXT_UNWRITTEN)
+			return -EINVAL;
+
+		error = libxfs_trans_get_buf(tp, mp->m_dev,
+				XFS_FSB_TO_DADDR(mp, map.br_startblock),
+				XFS_FSB_TO_BB(mp, map.br_blockcount),
+				0, &bp);
+		if (error)
+			break;
+		bp->b_ops = NULL;
+
+		count = min(len, XFS_FSB_TO_B(mp, map.br_blockcount));
+		memmove(bp->b_addr, buf, count);
+		bcount = BBTOB(bp->b_length);
+		if (count < bcount)
+			memset((char *)bp->b_addr + count, 0, bcount - count);
+
+		if (tp) {
+			libxfs_trans_log_buf(tp, bp, 0, bcount - 1);
+		} else {
+			libxfs_buf_mark_dirty(bp);
+			libxfs_buf_relse(bp);
+		}
+		if (error)
+			break;
+
+		buf += count;
+		len -= count;
+		bno += map.br_blockcount;
+	}
+
+	return error;
+}
diff --git a/mkfs/proto.c b/mkfs/proto.c
index 0cf2a84..71086bf 100644
--- a/mkfs/proto.c
+++ b/mkfs/proto.c
@@ -269,16 +269,12 @@
 {
 	struct xfs_bmbt_irec	map;
 	struct xfs_mount	*mp;
-	struct xfs_buf		*bp;
-	xfs_daddr_t		d;
 	xfs_extlen_t		nb;
 	int			nmap;
 	int			error;
 
 	mp = ip->i_mount;
 	if (len > 0) {
-		int	bcount;
-
 		nb = XFS_B_TO_FSB(mp, len);
 		nmap = 1;
 		error = -libxfs_bmapi_write(tp, ip, 0, nb, 0, nb, &map, &nmap);
@@ -288,30 +284,18 @@
 					progname);
 			exit(1);
 		}
-		if (error) {
+		if (error)
 			fail(_("error allocating space for a file"), error);
-		}
 		if (nmap != 1) {
 			fprintf(stderr,
 				_("%s: cannot allocate space for file\n"),
 				progname);
 			exit(1);
 		}
-		d = XFS_FSB_TO_DADDR(mp, map.br_startblock);
-		error = -libxfs_trans_get_buf(NULL, mp->m_dev, d,
-				nb << mp->m_blkbb_log, 0, &bp);
-		if (error) {
-			fprintf(stderr,
-				_("%s: cannot allocate buffer for file\n"),
-				progname);
-			exit(1);
-		}
-		memmove(bp->b_addr, buf, len);
-		bcount = BBTOB(bp->b_length);
-		if (len < bcount)
-			memset((char *)bp->b_addr + len, 0, bcount - len);
-		libxfs_buf_mark_dirty(bp);
-		libxfs_buf_relse(bp);
+
+		error = -libxfs_file_write(tp, ip, buf, len, false);
+		if (error)
+			fail(_("error writing file"), error);
 	}
 	ip->i_disk_size = len;
 }