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2015-04-22md/raid6 algorithms: xor_syndrome() for generic intMarkus Stockhausen1-1/+39
Start the algorithms with the very basic one. It is left and right optimized. That means we can avoid all calculations for unneeded pages above the right stop offset. For pages below the left start offset we still need the syndrome multiplication but without reading data pages. Signed-off-by: Markus Stockhausen <stockhausen@collogia.de> Signed-off-by: NeilBrown <neilb@suse.de>
2015-04-22md/raid6 algorithms: delta syndrome functionsMarkus Stockhausen1-1/+2
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 <stockhausen@collogia.de> Signed-off-by: NeilBrown <neilb@suse.de>
2011-10-20lib/raid6: Fix filename emitted in generated codeDan McGee1-1/+1
The files were renamed in commit cc4589ebf; fix the name in the file itself. Signed-off-by: Dan McGee <dpmcgee@gmail.com> Signed-off-by: NeilBrown <neilb@suse.de>
2010-08-11Rename raid6 files now they're in a 'raid6' directory.David Woodhouse1-0/+117
Linus asks 'why "raid6" twice?'. No reason. Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>