48 lines
2.7 KiB
ReStructuredText
48 lines
2.7 KiB
ReStructuredText
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==================
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Partial Parity Log
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==================
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Partial Parity Log (PPL) is a feature available for RAID5 arrays. The issue
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addressed by PPL is that after a dirty shutdown, parity of a particular stripe
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may become inconsistent with data on other member disks. If the array is also
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in degraded state, there is no way to recalculate parity, because one of the
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disks is missing. This can lead to silent data corruption when rebuilding the
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array or using it is as degraded - data calculated from parity for array blocks
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that have not been touched by a write request during the unclean shutdown can
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be incorrect. Such condition is known as the RAID5 Write Hole. Because of
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this, md by default does not allow starting a dirty degraded array.
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Partial parity for a write operation is the XOR of stripe data chunks not
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modified by this write. It is just enough data needed for recovering from the
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write hole. XORing partial parity with the modified chunks produces parity for
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the stripe, consistent with its state before the write operation, regardless of
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which chunk writes have completed. If one of the not modified data disks of
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this stripe is missing, this updated parity can be used to recover its
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contents. PPL recovery is also performed when starting an array after an
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unclean shutdown and all disks are available, eliminating the need to resync
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the array. Because of this, using write-intent bitmap and PPL together is not
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supported.
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When handling a write request PPL writes partial parity before new data and
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parity are dispatched to disks. PPL is a distributed log - it is stored on
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array member drives in the metadata area, on the parity drive of a particular
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stripe. It does not require a dedicated journaling drive. Write performance is
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reduced by up to 30%-40% but it scales with the number of drives in the array
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and the journaling drive does not become a bottleneck or a single point of
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failure.
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Unlike raid5-cache, the other solution in md for closing the write hole, PPL is
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not a true journal. It does not protect from losing in-flight data, only from
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silent data corruption. If a dirty disk of a stripe is lost, no PPL recovery is
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performed for this stripe (parity is not updated). So it is possible to have
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arbitrary data in the written part of a stripe if that disk is lost. In such
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case the behavior is the same as in plain raid5.
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PPL is available for md version-1 metadata and external (specifically IMSM)
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metadata arrays. It can be enabled using mdadm option --consistency-policy=ppl.
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There is a limitation of maximum 64 disks in the array for PPL. It allows to
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keep data structures and implementation simple. RAID5 arrays with so many disks
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are not likely due to high risk of multiple disks failure. Such restriction
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should not be a real life limitation.
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