B+-Tree Node (MSB+ Page)
All ReFS metadata is stored in B+-trees with the signature "MSB+". Each node occupies one metadata page: 16 KiB (4 consecutive clusters) on 4 KiB-cluster volumes, or 64 KiB (1 cluster) on 64 KiB-cluster volumes. An MSB+ page is built from four regions in order: the page header, a node header, the key-value data area, and a sorted key index.
B+-tree row header (16 bytes)
Each row in the data area begins with this header. The header points to where the key and value live within the row; both are measured from the start of the row.
| Offset | Size | Field | Description |
|---|---|---|---|
| 0x00 | 4 | Row size (u32) | Total byte size of the row, used as the stride to the next row. The value region is always last. (0x70 is the common value for a typical resident row, but it is a size, not a constant flag.) |
| 0x04 | 2 | Key offset (u16) | Byte offset from row start to key data |
| 0x06 | 2 | Key length (u16) | — |
| 0x08 | 2 | Reserved (u16) | — |
| 0x0A | 2 | Value offset (u16) | Byte offset from row start to value data |
| 0x0C | 2 | Value length (u16) | — |
| 0x0E | 2 | Reserved (u16) | — |
Rows are written sequentially in insertion order (not necessarily sorted); the sorted key index below restores order for lookups.
Page layout
1. Page header (80 bytes)
The common metadata page header shared with SUPB and CHKP. See Page Header.
Key MSB+-specific header fields:
- LCN slots 0–3 (offsets 0x20–0x38): cluster addresses of this page’s extent. On 4 KiB-cluster volumes, all four slots are used with consecutive values (
[self, self+1, self+2, self+3]). On 64 KiB-cluster volumes (where a page is 1 cluster), only slot 0 is meaningful. - Table OID (16-byte identifier at 0x40–0x4F): the high half at 0x40 (
TableIdHigh) is always 0 in practice, because table OIDs are small integers; the numeric table OID (e.g. 0x02 = Object ID Table, 0x0B = Container Table) isTableIdLowat 0x48 — the field the driver reads and compares. It is retained on orphaned/CoW-discarded pages, enabling identification.
2. Node header
A per-page node header (_SmsIndexHeader) follows the page header on every node, root and non-root alike. It describes that page’s own row set:
- Data-area boundaries (the row-data high-water cursor and free-space size)
- Row-pointer (key index) array boundaries
- Node type at +0x0C (0 = leaf, nonzero = internal/index node) and node flags at +0x0D (bit 0 set marks an index page)
- The per-node row/key count at +0x14 — the number of rows on this page (children for an inner node, leaf rows for a leaf). This is not the whole-table total.
On root nodes only, an index root descriptor (_SmsIndexRoot, Prade’s “Index Root”) precedes the node header at page+0x50, providing table-level metadata: the schema id, the leaf-extent count, and the total row count for the whole table (distinct from the per-node count above — on a multi-level tree they diverge cleanly). See the Schema Table for how the schema id selects key comparison rules.
3. Data area (key-value pairs)
Contains the actual row data: each row is the 16-byte row header above followed by its key and value bytes.
4. Key index (sorted offsets)
An array of offsets into the data area, maintained in sorted order according to the table’s key comparison rules (defined by the Schema Table). A binary search on this array locates a row without scanning the entire data area.
Inner vs leaf nodes
| Property | Inner Node | Leaf Node |
|---|---|---|
| Contains user data | No | Yes |
| Value content | Page references to child nodes | Actual key-value data |
| Key content | Separator keys (routing) | Full keys |
| Node-type byte (header +0x0C) | Nonzero | 0 |
Inner node values are page references (48, 72, or 104 bytes depending on checksum configuration) that point to child pages. The page reference format matches the one stored in the Checkpoint root pointer list.
Tree depth
Observed depths range from 1 (a root-only leaf, for small tables) to 2+ levels (a root inner node over leaf nodes). Tens of thousands of entries fit comfortably in a 2-level tree.
Orphaned pages
When copy-on-write replaces a page, the old page becomes orphaned but retains its original Table OID (the numeric value at header offset 0x48 = TableIdLow; the 0x40 high half is always 0). This enables forensic identification of discarded pages — they belong to a known table but are no longer reachable from the current checkpoint.
Driver functions
| Function | Purpose |
|---|---|
MsInsertRow | Inserts a key-value pair into a B+-tree. Handles page splits when a leaf is full. |
MsDeleteRow | Removes a key-value pair. Handles page merges when occupancy drops. |
MsLookupAllocation | Queries allocation state via B+-tree lookup. Core read path. |
EnumerateBPlusTable | Iterates all entries in a B+-tree in key order. |
MsAllocateRowWithBuffer | Allocates a B+-tree row with a pre-sized buffer for the value. |
Cross-references
- Page Header — the 80-byte header shared by all metadata pages
- Page References — the format used in inner node values
- Schema Table — defines key comparison rules per table type
- Checkpoint (CHKP) — root pointers to the top-level B+-trees
Evidence
The B+-tree storage engine and the MSB+ page model are confirmed in the driver (the CmsBPlusTable / CmsTable classes) and raw-disk verified across the corpus (every metadata page carries the MSB+ signature). The row header, node header, and index-root descriptor offsets are decompiled-confirmed and re-measured on disk: the node-type byte and the per-node count at header +0x14 hold on every leaf/inner page measured, and the index-root total-row count tracks an independent leaf-walk. The 0x40/0x48 table-OID split (always-0 high half, numeric low half) is the same. The insert/delete/lookup/enumerate/allocate functions are PDB symbols in the driver.