[PATCH 03/20] drbd: introduce DRBD 9 on-disk metadata format

Christoph Böhmwalder
2026-03-27T22:38:55Z

Add a new header that captures the DRBD 9 on-disk metadata layout,
enabling state tracking for mutiple peers.
It includes the per-device superblock and per-peer slot structures
needed to track bitmap UUIDs and sync state for each peer.

Co-developed-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Co-developed-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Co-developed-by: Joel Colledge <joel.colledge@linbit.com>
Signed-off-by: Joel Colledge <joel.colledge@linbit.com>
Co-developed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
---
drivers/block/drbd/drbd_meta_data.h | 126 ++++++++++++++++++++++++++++
1 file changed, 126 insertions(+)
create mode 100644 drivers/block/drbd/drbd_meta_data.h

diff --git a/drivers/block/drbd/drbd_meta_data.h b/drivers/block/drbd/drbd_meta_data.h
new file mode 100644
index 000000000000..af77e8d53f02
--- /dev/null
+++ b/drivers/block/drbd/drbd_meta_data.h
@@ -0,0 +1,126 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef DRBD_META_DATA_H
+#define DRBD_META_DATA_H
+
+/* how I came up with this magic?
+ * base64 decode "actlog==" ;) */
+#define DRBD_AL_MAGIC 0x69cb65a2
+
+#define BM_BLOCK_SHIFT_4k 12 /* 4k per bit */
+#define BM_BLOCK_SHIFT_MIN BM_BLOCK_SHIFT_4k
+#define BM_BLOCK_SHIFT_MAX 20
+#define BM_BLOCK_SIZE_4k 4096
+#define BM_BLOCK_SIZE_MIN (1<<BM_BLOCK_SHIFT_MIN)
+#define BM_BLOCK_SIZE_MAX (1<<BM_BLOCK_SHIFT_MAX)
+
+struct peer_dev_md_on_disk_9 {
+ __be64 bitmap_uuid;
+ __be64 bitmap_dagtag;
+ __be32 flags;
+ __be32 bitmap_index;
+ __be32 reserved_u32[2];
+} __packed;
+
+struct meta_data_on_disk_9 {
+ __be64 effective_size; /* last agreed size */
+ __be64 current_uuid;
+ __be64 members; /* only if MDF_HAVE_MEMBERS_MASK is in the flags */
+ __be64 reserved_u64[3]; /* to have the magic at the same position as in v07, and v08 */
+ __be64 device_uuid;
+ __be32 flags; /* MDF */
+ __be32 magic;
+ __be32 md_size_sect;
+ __be32 al_offset; /* offset to this block */
+ __be32 al_nr_extents; /* important for restoring the AL */
+ __be32 bm_offset; /* offset to the bitmap, from here */
+ __be32 bm_bytes_per_bit; /* BM_BLOCK_SIZE */
+ __be32 la_peer_max_bio_size; /* last peer max_bio_size */
+ __be32 bm_max_peers;
+ __be32 node_id;
+
+ /* see al_tr_number_to_on_disk_sector() */
+ __be32 al_stripes;
+ __be32 al_stripe_size_4k;
+
+ __be32 reserved_u32[2];
+
+ struct peer_dev_md_on_disk_9 peers[DRBD_PEERS_MAX];
+ __be64 history_uuids[HISTORY_UUIDS];
+
+ unsigned char padding_start[0];
+ unsigned char padding_end[0] __aligned(4096);
+} __packed;
+
+/* Attention, these two are defined in drbd_int.h as well! */
+#define AL_UPDATES_PER_TRANSACTION 64
+#define AL_CONTEXT_PER_TRANSACTION 919
+
+enum al_transaction_types {
+ AL_TR_UPDATE = 0,
+ AL_TR_INITIALIZED = 0xffff
+};
+/* all fields on disc in big endian */
+struct __packed al_transaction_on_disk {
+ /* don't we all like magic */
+ __be32 magic;
+
+ /* to identify the most recent transaction block
+ * in the on disk ring buffer */
+ __be32 tr_number;
+
+ /* checksum on the full 4k block, with this field set to 0. */
+ __be32 crc32c;
+
+ /* type of transaction, special transaction types like:
+ * purge-all, set-all-idle, set-all-active, ... to-be-defined
+ * see also enum al_transaction_types */
+ __be16 transaction_type;
+
+ /* we currently allow only a few thousand extents,
+ * so 16bit will be enough for the slot number. */
+
+ /* how many updates in this transaction */
+ __be16 n_updates;
+
+ /* maximum slot number, "al-extents" in drbd.conf speak.
+ * Having this in each transaction should make reconfiguration
+ * of that parameter easier. */
+ __be16 context_size;
+
+ /* slot number the context starts with */
+ __be16 context_start_slot_nr;
+
+ /* Some reserved bytes. Expected usage is a 64bit counter of
+ * sectors-written since device creation, and other data generation tag
+ * supporting usage */
+ __be32 __reserved[4];
+
+ /* --- 36 byte used --- */
+
+ /* Reserve space for up to AL_UPDATES_PER_TRANSACTION changes
+ * in one transaction, then use the remaining byte in the 4k block for
+ * context information. "Flexible" number of updates per transaction
+ * does not help, as we have to account for the case when all update
+ * slots are used anyways, so it would only complicate code without
+ * additional benefit.
+ */
+ __be16 update_slot_nr[AL_UPDATES_PER_TRANSACTION];
+
+ /* but the extent number is 32bit, which at an extent size of 4 MiB
+ * allows to cover device sizes of up to 2**54 Byte (16 PiB) */
+ __be32 update_extent_nr[AL_UPDATES_PER_TRANSACTION];
+
+ /* --- 420 bytes used (36 + 64*6) --- */
+
+ /* 4096 - 420 = 3676 = 919 * 4 */
+ __be32 context[AL_CONTEXT_PER_TRANSACTION];
+};
+
+#define DRBD_AL_PMEM_MAGIC 0x6aa667a6 /* "al==pmem" */
+
+struct __packed al_on_pmem {
+ __be32 magic;
+ __be32 slots[];
+};
+
+#endif
--
2.53.0