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- ==================
- 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.
- There is a limitation of maximum 64 disks in the array for PPL. It allows to
- keep data structures and implementation simple. RAID5 arrays with so many disks
- are not likely due to high risk of multiple disks failure. Such restriction
- should not be a real life limitation.
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