From fe5cbc6e06c7d8b3a86f6f5491d74766bb5c2827 Mon Sep 17 00:00:00 2001 From: Markus Stockhausen Date: Mon, 15 Dec 2014 12:57:04 +1100 Subject: md/raid6 algorithms: delta syndrome functions 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/int.uc | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'lib/raid6/int.uc') diff --git a/lib/raid6/int.uc b/lib/raid6/int.uc index 5b50f8d..5ca60be 100644 --- a/lib/raid6/int.uc +++ b/lib/raid6/int.uc @@ -109,7 +109,8 @@ static void raid6_int$#_gen_syndrome(int disks, size_t bytes, void **ptrs) const struct raid6_calls raid6_intx$# = { raid6_int$#_gen_syndrome, - NULL, /* always valid */ + NULL, /* XOR not yet implemented */ + NULL, /* always valid */ "int" NSTRING "x$#", 0 }; -- cgit v1.1