Load the log from each disk when starting the array and recover if the
array is dirty.
The initial empty PPL is written by mdadm. When loading the log we
verify the header checksum and signature. For external metadata arrays
the signature is verified in userspace, so here we read it from the
header, verifying only if it matches on all disks, and use it later when
writing PPL.
In addition to the header checksum, each header entry also contains a
checksum of its partial parity data. If the header is valid, recovery is
performed for each entry until an invalid entry is found. If the array
is not degraded and recovery using PPL fully succeeds, there is no need
to resync the array because data and parity will be consistent, so in
this case resync will be disabled.
Due to compatibility with IMSM implementations on other systems, we
can't assume that the recovery data block size is always 4K. Writes
generated by MD raid5 don't have this issue, but when recovering PPL
written in other environments it is possible to have entries with
512-byte sector granularity. The recovery code takes this into account
and also the logical sector size of the underlying drives.
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>
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>
