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.

APArtur Paszkiewicz committed 9 years ago