From 3418d036c81dcb604b7c7c71b209d5890a8418aa Mon Sep 17 00:00:00 2001 From: Artur Paszkiewicz Date: Thu, 9 Mar 2017 09:59:59 +0100 Subject: raid5-ppl: Partial Parity Log write logging implementation Implement the calculation of partial parity for a stripe and PPL write logging functionality. The description of PPL is added to the documentation. More details can be found in the comments in raid5-ppl.c. Attach a page for holding the partial parity data to stripe_head. Allocate it only if mddev has the MD_HAS_PPL flag set. Partial parity is the xor of not modified data chunks of a stripe and is calculated as follows: - reconstruct-write case: xor data from all not updated disks in a stripe - read-modify-write case: xor old data and parity from all updated disks in a stripe Implement it using the async_tx API and integrate into raid_run_ops(). It must be called when we still have access to old data, so do it when STRIPE_OP_BIODRAIN is set, but before ops_run_prexor5(). The result is stored into sh->ppl_page. Partial parity is not meaningful for full stripe write and is not stored in the log or used for recovery, so don't attempt to calculate it when stripe has STRIPE_FULL_WRITE. Put the PPL metadata structures to md_p.h because userspace tools (mdadm) will also need to read/write PPL. Warn about using PPL with enabled disk volatile write-back cache for now. It can be removed once disk cache flushing before writing PPL is implemented. Signed-off-by: Artur Paszkiewicz Signed-off-by: Shaohua Li --- Documentation/md/raid5-ppl.txt | 44 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) create mode 100644 Documentation/md/raid5-ppl.txt (limited to 'Documentation/md/raid5-ppl.txt') diff --git a/Documentation/md/raid5-ppl.txt b/Documentation/md/raid5-ppl.txt new file mode 100644 index 000000000000..127072b09363 --- /dev/null +++ b/Documentation/md/raid5-ppl.txt @@ -0,0 +1,44 @@ +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. -- cgit v1.2.3