• Artur Paszkiewicz's avatar
    raid5-ppl: Partial Parity Log write logging implementation · 3418d036
    Artur Paszkiewicz authored
    
    
    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: default avatarArtur Paszkiewicz <artur.paszkiewicz@intel.com>
    Signed-off-by: default avatarShaohua Li <shli@fb.com>
    3418d036