diff --git a/utils/src/format.h b/utils/src/format.h
index a80e95b0..6f80059e 100644
--- a/utils/src/format.h
+++ b/utils/src/format.h
@@ -19,6 +19,7 @@
  */
 #define SCOUTFS_SUPER_BLKNO ((64 * 1024) >> SCOUTFS_BLOCK_SHIFT)
 #define SCOUTFS_SUPER_NR 2
+#define SCOUTFS_BUDDY_BLKNO (SCOUTFS_SUPER_BLKNO + SCOUTFS_SUPER_NR)
 
 /*
  * This header is found at the start of every block so that we can
@@ -102,6 +103,38 @@ struct scoutfs_btree_item {
 
 #define SCOUTFS_UUID_BYTES 16
 
+/*
+ * Arbitrarily choose a reasonably fine grained 64byte chunk.  This is a
+ * balance between write amplification of writing chunks with a single
+ * modified bit, storage overhead of partial blocks losing a chunk to
+ * make room for the block header and having a pos field per chunk, and
+ * runtime memory overhead of a bit per chunk.
+ */
+#define SCOUTFS_BUDDY_CHUNK_LE64S 8
+#define SCOUTFS_BUDDY_CHUNK_BYTES (SCOUTFS_BUDDY_CHUNK_LE64S * 8)
+#define SCOUTFS_BUDDY_CHUNK_BITS (SCOUTFS_BUDDY_CHUNK_BYTES * 8)
+
+/*
+ * After the pair of super blocks are a preallocated ring of blocks
+ * which record modified regions of the buddy bitmap allocator.
+ *
+ * The seq's header needs to match the unwrapped ring index of the
+ * block.
+ */
+struct scoutfs_buddy_block {
+	struct scoutfs_block_header hdr;
+	u8 nr_chunks;
+	struct scoutfs_buddy_chunk {
+		__le32 pos;
+		__le64 bits[SCOUTFS_BUDDY_CHUNK_LE64S];
+	} __packed chunks[0];
+} __packed;
+
+#define SCOUTFS_BUDDY_CHUNKS_PER_BLOCK \
+	((SCOUTFS_BLOCK_SIZE - offsetof(struct scoutfs_buddy_block, chunks)) /\
+	 SCOUTFS_BUDDY_CHUNK_BYTES)
+
+
 /*
  * The super is stored in a pair of blocks in the first chunk on the
  * device.
@@ -116,6 +149,11 @@ struct scoutfs_super_block {
 	struct scoutfs_block_header hdr;
 	__le64 id;
 	__u8 uuid[SCOUTFS_UUID_BYTES];
+	__le64 total_blocks;
+	__le32 buddy_blocks;
+	__le32 buddy_sweep_bit;
+	__le64 buddy_head;
+	__le64 buddy_tail;
         struct scoutfs_btree_root btree_root;
 } __packed;
 
diff --git a/utils/src/mkfs.c b/utils/src/mkfs.c
index ac0c196e..116e8abf 100644
--- a/utils/src/mkfs.c
+++ b/utils/src/mkfs.c
@@ -39,6 +39,24 @@ static int write_block(int fd, u64 blkno, struct scoutfs_block_header *hdr)
 	return 0;
 }
 
+/*
+ * Calculate the number of buddy blocks that are needed to track the
+ * allocation of a device with the given byte size.  We need an even
+ * number of buddy blocks that contain 8 bits for every device block.  This
+ * is a bit overly conservative in that it doesn't subtract the buddy
+ * blocks and super block from the calculation.
+ */
+static u32 calc_buddy_blocks(u64 total_blocks)
+{
+	u64 buddy_bits = total_blocks * 8;
+	u64 chunks = DIV_ROUND_UP(buddy_bits, SCOUTFS_BUDDY_CHUNK_BITS);
+	u64 blocks = DIV_ROUND_UP(chunks, SCOUTFS_BUDDY_CHUNKS_PER_BLOCK);
+
+	/* XXX check u32 overflow? */
+
+	return round_up(blocks, 2);
+}
+
 static int write_new_fs(char *path, int fd)
 {
 	struct scoutfs_super_block *super;
@@ -51,6 +69,8 @@ static int write_new_fs(char *path, int fd)
 	unsigned int i;
 	u64 size;
 	u64 blkno;
+	u64 total_blocks;
+	u64 buddy_blocks;
 	void *buf;
 	int ret;
 
@@ -72,6 +92,15 @@ static int write_new_fs(char *path, int fd)
 		goto out;
 	}
 
+	/* the block limit is totally arbitrary */
+	total_blocks = size / SCOUTFS_BLOCK_SIZE;
+	if (total_blocks < 32) {
+		fprintf(stderr, "%llu byte device only has room for %llu %u byte blocks, needs at least 32 blocks\n",
+			size, total_blocks, SCOUTFS_BLOCK_SIZE);
+		goto out;
+	}
+	buddy_blocks = calc_buddy_blocks(total_blocks);
+
 	root_key.inode = cpu_to_le64(SCOUTFS_ROOT_INO);
 	root_key.type = SCOUTFS_INODE_KEY;
 	root_key.offset = 0;
@@ -85,6 +114,8 @@ static int write_new_fs(char *path, int fd)
 	super->hdr.seq = cpu_to_le64(1);
 	super->id = cpu_to_le64(SCOUTFS_SUPER_ID);
 	uuid_generate(super->uuid);
+	super->total_blocks = cpu_to_le64(total_blocks);
+	super->buddy_blocks = cpu_to_le32(buddy_blocks);
 
 	/* write a btree leaf root inode item */
 	memset(buf, 0, SCOUTFS_BLOCK_SIZE);
@@ -142,10 +173,12 @@ static int write_new_fs(char *path, int fd)
 	uuid_unparse(super->uuid, uuid_str);
 
 	printf("Created scoutfs filesystem:\n"
-	       "  block size: %u\n"
+	       "  total blocks: %llu\n"
+	       "  buddy blocks: %llu\n"
 	       "  fsid: %llx\n"
 	       "  uuid: %s\n",
-		SCOUTFS_BLOCK_SIZE, le64_to_cpu(super->hdr.fsid), uuid_str);
+		total_blocks, buddy_blocks,
+		le64_to_cpu(super->hdr.fsid), uuid_str);
 
 	ret = 0;
 out:
diff --git a/utils/src/print.c b/utils/src/print.c
index 3aa6277d..36097cfe 100644
--- a/utils/src/print.c
+++ b/utils/src/print.c
@@ -178,6 +178,49 @@ static int print_btree_block(int fd, __le64 blkno, u8 level)
 	return ret;
 }
 
+static int print_buddy_blocks(int fd, struct scoutfs_super_block *super)
+{
+	struct scoutfs_buddy_chunk *chunk;
+	struct scoutfs_buddy_block *bb;
+	u64 blkno;
+	u64 blocks;
+	u64 head;
+	u64 tail;
+	int i;
+	int j;
+
+	blocks = le32_to_cpu(super->buddy_blocks);
+	head = le64_to_cpu(super->buddy_head);
+	tail = le64_to_cpu(super->buddy_tail);
+
+	/* XXX make sure values are sane */
+
+	for (; head < tail; head++) {
+
+		blkno = SCOUTFS_BUDDY_BLKNO + (head % blocks);
+		bb = read_block(fd, blkno);
+		if (!bb)
+			return -ENOMEM;
+
+		printf("buddy blkno %llu\n", blkno);
+		print_block_header(&bb->hdr);
+		printf("  nr_chunks %u\n", bb->nr_chunks);
+		for (i = 0; i < bb->nr_chunks; i++) {
+			chunk = &bb->chunks[i];
+
+			printf("   [%u]: pos %u bits ",
+				i, le32_to_cpu(chunk->pos));
+			for (j = 0; j < SCOUTFS_BUDDY_CHUNK_LE64S; j++)
+				printf("%016llx", le64_to_cpu(chunk->bits[j]));
+			printf("\n");
+		}
+
+		free(bb);
+	}
+
+	return 0;
+}
+
 static int print_super_blocks(int fd)
 {
 	struct scoutfs_super_block *super;
@@ -198,6 +241,13 @@ static int print_super_blocks(int fd)
 		print_block_header(&super->hdr);
 		printf("  id %llx uuid %s\n",
 		       le64_to_cpu(super->id), uuid_str);
+		printf("  total_blocks %llu buddy_blocks %u buddy_sweep_bit %u\n"
+		       "  buddy_head %llu buddy_tail %llu\n",
+			le64_to_cpu(super->total_blocks),
+			le32_to_cpu(super->buddy_blocks),
+			le32_to_cpu(super->buddy_sweep_bit),
+			le64_to_cpu(super->buddy_head),
+			le64_to_cpu(super->buddy_tail));
 		printf("  btree_root: height %u seq %llu blkno %llu\n",
 			super->btree_root.height,
 			le64_to_cpu(super->btree_root.ref.seq),
@@ -211,11 +261,15 @@ static int print_super_blocks(int fd)
 
 	super = &recent;
 
-	if (super->btree_root.height)
+	ret = print_buddy_blocks(fd, super);
+
+	if (super->btree_root.height) {
 		err = print_btree_block(fd, super->btree_root.ref.blkno,
 					super->btree_root.height - 1);
-	if (err && !ret)
-		ret = err;
+		if (err && !ret)
+			ret = err;
+	}
+
 	return ret;
 }