diff --git a/bitmap.h b/bitmap.h
index f7fa938..ba08931 100644
--- a/bitmap.h
+++ b/bitmap.h
@@ -40,14 +40,28 @@ static inline uint32_t get_free_inode(struct simplefs_sb_info *sbi)
 }
 
 /* Return 'len' unused block(s) number and mark it used.
+ * Clean the block content.
  * Return 0 if no enough free block(s) were found.
  */
-static inline uint32_t get_free_blocks(struct simplefs_sb_info *sbi,
-                                       uint32_t len)
+static inline uint32_t get_free_blocks(struct super_block *sb, uint32_t len)
 {
+    struct simplefs_sb_info *sbi = SIMPLEFS_SB(sb);
     uint32_t ret = get_first_free_bits(sbi->bfree_bitmap, sbi->nr_blocks, len);
-    if (ret)
-        sbi->nr_free_blocks -= len;
+    if (!ret) /* No enough free blocks */
+        return 0;
+
+    sbi->nr_free_blocks -= len;
+    for (uint32_t i = 0; i < len; i++) {
+        struct buffer_head *bh = sb_bread(sb, ret + i);
+        if (!bh) {
+            pr_err("get_free_blocks: sb_bread failed for block %d\n", ret + i);
+            sbi->nr_free_blocks += len;
+            return -EIO;
+        }
+        memset(bh->b_data, 0, SIMPLEFS_BLOCK_SIZE);
+        mark_buffer_dirty(bh);
+        brelse(bh);
+    }
     return ret;
 }
 
diff --git a/file.c b/file.c
index 67a8e82..a26a12e 100644
--- a/file.c
+++ b/file.c
@@ -20,7 +20,6 @@ static int simplefs_file_get_block(struct inode *inode,
                                    int create)
 {
     struct super_block *sb = inode->i_sb;
-    struct simplefs_sb_info *sbi = SIMPLEFS_SB(sb);
     struct simplefs_inode_info *ci = SIMPLEFS_INODE(inode);
     struct simplefs_file_ei_block *index;
     struct buffer_head *bh_index;
@@ -52,7 +51,7 @@ static int simplefs_file_get_block(struct inode *inode,
             ret = 0;
             goto brelse_index;
         }
-        bno = get_free_blocks(sbi, 8);
+        bno = get_free_blocks(sb, 8);
         if (!bno) {
             ret = -ENOSPC;
             goto brelse_index;
diff --git a/inode.c b/inode.c
index 5750016..ef60889 100644
--- a/inode.c
+++ b/inode.c
@@ -248,7 +248,7 @@ static struct inode *simplefs_new_inode(struct inode *dir, mode_t mode)
     ci = SIMPLEFS_INODE(inode);
 
     /* Get a free block for this new inode's index */
-    bno = get_free_blocks(sbi, 1);
+    bno = get_free_blocks(sb, 1);
     if (!bno) {
         ret = -ENOSPC;
         goto put_inode;
@@ -381,7 +381,7 @@ static int simplefs_create(struct inode *dir,
     fi = eblock->nr_files % SIMPLEFS_FILES_PER_BLOCK;
 
     if (!eblock->extents[ei].ee_start) {
-        bno = get_free_blocks(SIMPLEFS_SB(sb), 8);
+        bno = get_free_blocks(sb, 8);
         if (!bno) {
             ret = -ENOSPC;
             goto iput;
@@ -753,7 +753,7 @@ static int simplefs_rename(struct inode *old_dir,
     /* insert in new parent directory */
     /* Get new freeblocks for extent if needed*/
     if (new_pos < 0) {
-        bno = get_free_blocks(SIMPLEFS_SB(sb), 8);
+        bno = get_free_blocks(sb, 8);
         if (!bno) {
             ret = -ENOSPC;
             goto release_new;
@@ -909,7 +909,7 @@ static int simplefs_link(struct dentry *old_dentry,
     fi = eblock->nr_files % SIMPLEFS_FILES_PER_BLOCK;
 
     if (eblock->extents[ei].ee_start == 0) {
-        bno = get_free_blocks(SIMPLEFS_SB(sb), 8);
+        bno = get_free_blocks(sb, 8);
         if (!bno) {
             ret = -ENOSPC;
             goto end;
@@ -1002,7 +1002,7 @@ static int simplefs_symlink(struct inode *dir,
     fi = eblock->nr_files % SIMPLEFS_FILES_PER_BLOCK;
 
     if (eblock->extents[ei].ee_start == 0) {
-        bno = get_free_blocks(SIMPLEFS_SB(sb), 8);
+        bno = get_free_blocks(sb, 8);
         if (!bno) {
             ret = -ENOSPC;
             goto end;
diff --git a/mkfs.c b/mkfs.c
index f0af711..01a6e0b 100644
--- a/mkfs.c
+++ b/mkfs.c
@@ -235,7 +235,20 @@ static int write_bfree_blocks(int fd, struct superblock *sb)
 
 static int write_data_blocks(int fd, struct superblock *sb)
 {
-    /* FIXME: unimplemented */
+    char *buffer = calloc(1, SIMPLEFS_BLOCK_SIZE);
+    if (!buffer) {
+        perror("Failed to allocate memory");
+        return -1;
+    }
+
+    ssize_t ret = write(fd, buffer, SIMPLEFS_BLOCK_SIZE);
+    if (ret != SIMPLEFS_BLOCK_SIZE) {
+        perror("Failed to write data block");
+        free(buffer);
+        return -1;
+    }
+
+    free(buffer);
     return 0;
 }
 
@@ -315,7 +328,7 @@ int main(int argc, char **argv)
         goto free_sb;
     }
 
-    /* Write data blocks */
+    /* clear a root index block */
     ret = write_data_blocks(fd, sb);
     if (ret) {
         perror("write_data_blocks():");