Constant ROWS_PER_ROW_GROUP 

pub(crate) const ROWS_PER_ROW_GROUP: usize = _; // 1_048_576usize

Rows buffered before they are written as one row group (1,048,576).

One row group normally holds a whole rotation period (600,000 rows for five minutes at 2 kHz). Large row groups let dictionary and delta encoding work across the file instead of restarting every 1,000 rows. An unfinished file has no footer and is quarantined whole, so smaller row groups would not make buffered rows any safer; unacked JetStream messages are what protect them.