Partial Parity Log
  
  Partial Parity Log (PPL) is a feature available for RAID5 arrays. The issue
  addressed by PPL is that after a dirty shutdown, parity of a particular stripe
  may become inconsistent with data on other member disks. If the array is also
  in degraded state, there is no way to recalculate parity, because one of the
  disks is missing. This can lead to silent data corruption when rebuilding the
  array or using it is as degraded - data calculated from parity for array blocks
  that have not been touched by a write request during the unclean shutdown can
  be incorrect. Such condition is known as the RAID5 Write Hole. Because of
  this, md by default does not allow starting a dirty degraded array.
  
  Partial parity for a write operation is the XOR of stripe data chunks not
  modified by this write. It is just enough data needed for recovering from the
  write hole. XORing partial parity with the modified chunks produces parity for
  the stripe, consistent with its state before the write operation, regardless of
  which chunk writes have completed. If one of the not modified data disks of
  this stripe is missing, this updated parity can be used to recover its
  contents. PPL recovery is also performed when starting an array after an
  unclean shutdown and all disks are available, eliminating the need to resync
  the array. Because of this, using write-intent bitmap and PPL together is not
  supported.
  
  When handling a write request PPL writes partial parity before new data and
  parity are dispatched to disks. PPL is a distributed log - it is stored on
  array member drives in the metadata area, on the parity drive of a particular
  stripe.  It does not require a dedicated journaling drive. Write performance is
  reduced by up to 30%-40% but it scales with the number of drives in the array
  and the journaling drive does not become a bottleneck or a single point of
  failure.
  
  Unlike raid5-cache, the other solution in md for closing the write hole, PPL is
  not a true journal. It does not protect from losing in-flight data, only from
  silent data corruption. If a dirty disk of a stripe is lost, no PPL recovery is
  performed for this stripe (parity is not updated). So it is possible to have
  arbitrary data in the written part of a stripe if that disk is lost. In such
  case the behavior is the same as in plain raid5.
  
  PPL is available for md version-1 metadata and external (specifically IMSM)
  metadata arrays. It can be enabled using mdadm option --consistency-policy=ppl.
  
  Currently, volatile write-back cache should be disabled on all member drives
  when using PPL. Otherwise it cannot guarantee consistency in case of power
  failure.