| 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. |