/sys/fs/btrfs/<uuid>/stripe_park_stats reports lifetime counters for
the parking machinery: rbios parked, bytes merged into parked rbios,
and how each park ended (filled to a full stripe, kicked by a waiter or
a settle/retire flush, or expired at its deadline), plus how many
flushed writes were padded to full stripes versus still needing the
RMW read phase. The filled/kicked/expired split shows directly whether
the deadlines are sized right for a given system -- expired parks that
later reappear as rmw_reads are the batching the window failed to
capture -- so any future retuning (or an adaptive deadline) can argue
from measurements instead of taste.
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