Module layout 

On-disk layout of the double-write buffer file.

offset 0                       one aligned block: file header
  [magic:4][count:4][write_pos:4][zero padding to DWB_HEADER_STRIDE]

offset DWB_HEADER_STRIDE + i * DWB_SLOT_STRIDE      slot i
  [slot_magic:4][seq:8][total_size:4][prefix_crc:4][record header][payload]

Slot sequence numbers

The ring is fixed-size, so one LSN can legitimately occupy two slots: a crash between the DWB fsync and the WAL fsync leaves an LSN durable in the DWB but absent from the WAL, recovery resumes at that LSN, and the next record written under it lands in a different slot. Both copies pass CRC, so nothing in the record itself says which one the WAL actually committed. seq breaks that tie: it increases with every slot write, so the highest seq is always the copy written last. Without it, recovery can hand back a payload that was never acknowledged to any client.

Format change

Slots begin with slot_magic, not a bare [total_size:4]. Its value is far outside the range a total_size could ever hold, so a slot written by an older build that used the bare format fails the prefix check and is treated as unusable rather than being misread as a sequence-numbered slot. A DWB only ever holds records from the tail of the log — the worst case is that torn-write recovery falls back to “no copy available” for records written before the upgrade.

Functions

slot_record_max

Largest record — WAL header plus payload — the buffer can mirror. Records above this size are appended to the WAL without torn-write protection.

slot_stride

Slot stride in bytes. Exposed for tests and for callers that want to size DWB files ahead of time.