md/raid6 algorithms: delta syndrome functions

2015-04-21T22:00:41Z

v3: s-o-b comment, explanation of performance and descision for the start/stop implementation Implementing rmw functionality for RAID6 requires optimized syndrome calculation. Up to now we can only generate a complete syndrome. The target P/Q pages are always overwritten. With this patch we provide a framework for inplace P/Q modification. In the first place simply fill those functions with NULL values. xor_syndrome() has two additional parameters: start & stop. These will indicate the first and last page that are changing during a rmw run. That makes it possible to avoid several unneccessary loops and speed up calculation. The caller needs to implement the following logic to make the functions work. 1) xor_syndrome(disks, start, stop, ...): "Remove" all data of source blocks inside P/Q between (and including) start and end. 2) modify any block with start <= block <= stop 3) xor_syndrome(disks, start, stop, ...): "Reinsert" all data of source blocks into P/Q between (and including) start and end. Pages between start and stop that won't be changed should be filled with a pointer to the kernel zero page. The reasons for not taking NULL pages are: 1) Algorithms cross the whole source data line by line. Thus avoid additional branches. 2) Having a NULL page avoids calculating the XOR P parity but still need calulation steps for the Q parity. Depending on the algorithm unrolling that might be only a difference of 2 instructions per loop. The benchmark numbers of the gen_syndrome() functions are displayed in the kernel log. Do the same for the xor_syndrome() functions. This will help to analyze performance problems and give an rough estimate how well the algorithm works. The choice of the fastest algorithm will still depend on the gen_syndrome() performance. With the start/stop page implementation the speed can vary a lot in real life. E.g. a change of page 0 & page 15 on a stripe will be harder to compute than the case where page 0 & page 1 are XOR candidates. To be not to enthusiatic about the expected speeds we will run a worse case test that simulates a change on the upper half of the stripe. So we do: 1) calculation of P/Q for the upper pages 2) continuation of Q for the lower (empty) pages Signed-off-by: Markus Stockhausen Signed-off-by: NeilBrown

lib/raid6: build proper files on corresponding arch

2012-12-13T08:51:04Z

sse and avx2 stuff only exist on x86 arch, and we don't need to build altivec on x86. And we can do that at lib/raid6/Makefile. Proposed-by: H. Peter Anvin Signed-off-by: Yuanhan Liu Reviewed-by: H. Peter Anvin Signed-off-by: Jim Kukunas Signed-off-by: NeilBrown

Remove all #inclusions of asm/system.h

2012-03-28T17:30:03Z

Remove all #inclusions of asm/system.h preparatory to splitting and killing it. Performed with the following command: perl -p -i -e 's!^#\s*include\s*.*\n!!' `grep -Irl '^#\s*include\s*' *` Signed-off-by: David Howells

Disintegrate asm/system.h for PowerPC

2012-03-28T17:30:02Z

Disintegrate asm/system.h for PowerPC. Signed-off-by: David Howells Acked-by: Benjamin Herrenschmidt cc: linuxppc-dev@lists.ozlabs.org

Rename raid6 files now they're in a 'raid6' directory.

2010-08-10T23:19:05Z

Linus asks 'why "raid6" twice?'. No reason. Signed-off-by: David Woodhouse

user/sven/linux.git/lib/raid6/altivec.uc, branch v4.4.76

md/raid6 algorithms: delta syndrome functions

lib/raid6: build proper files on corresponding arch

Remove all #inclusions of asm/system.h

Disintegrate asm/system.h for PowerPC

Rename raid6 files now they're in a 'raid6' directory.