linux/Documentation/md/raid5-ppl.txt

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.
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 <artur.paszkiewicz@intel.com> Signed-off-by: Shaohua Li <shli@fb.com> 2017-03-09 08:59:59 +00:00			`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.`