| ================== | |
| 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. |