Constant WAL_SYNC_PERIOD 

pub const WAL_SYNC_PERIOD: Duration;

How long an acknowledged export can sit in the page cache before it is on the device.

This is the entire power-loss exposure window, and it is a constant for the same reason max_block_age is: the operator cannot price the trade without knowing what an fsync costs on their volume, and the engine measures that every time it does one. On this machine F_FULLFSYNC is ~4 ms (section 10), so a quarter-second period spends under 2% of one thread and bounds the loss at a quarter second of ingest. Shorter buys very little — the exposure is already smaller than a Collector’s own batch timeout, so the exporter is holding more unsent data than this window holds unsynced.