45 lines
2.6 KiB
Plaintext
45 lines
2.6 KiB
Plaintext
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.
|