btrfs: raid56: pad sub-stripe writes to full stripes in open runs
A sub-stripe write does a full RMW: read every untouched data sector of
the stripe, recompute parity, write. Under stripe-exclusive allocation
the read phase is usually pointless: a stripe covered by a live stripe
run has never been written at or past the run's allocation frontier, so
the sectors being read contain nothing.
When every data sector the rbio does not cover lies at or past the
frontier, zero-fill those sectors instead of reading them and write
them out with the stripe -- the zeros must reach the disk, or the
parity (computed over them) would not match what scrub reads back. The
partial write becomes one full-stripe write: no read phase, one parity
pass. A frontier that grows during the attempt is safe: the newer
allocation's write serializes behind this rbio's stripe lock and lands
over the zeros. If any uncovered sector is below the frontier (already
allocated to someone else), fall back to the normal RMW.
Together with kicking parked rbios before the fast fsync's writeback
wait, this removes both stalls the raid56 layer added to fsync under
stripe_alloc: the park deadline and the RMW read round trip.
projects / linux / commit