v2: reordering and merging of patches as Neil requested. More verification & benchmark numbers Once again thanks to an older patch from Kumar Sundararajan and Dan Williams that helped me to understand RAID6 logic inside md better. Everything is based on ideas & discussions that started with http://marc.info/?l=linux-raid&m=136624783417452&w=1 Another try to implement RMW support for RAID6. This time improve syndrome calculation too. A few things to note: 1) Patches are based on official 3.16 kernel git. 2) The required optimized syndrome functions were implemented if possible. Generic & SSE2 are the ones that I could write & test on my machine. If you want to test/benchmark this patch ensure that you force select one of the two. Programmers with appropriate hardware in their hands are encouraged to send the missing algorithms. 3) raid6 test program was enhanced to verify algorithm correctness. Additionaly this release was checked with a self written single threaded test tool I called wprd (write predictable random data). Checked features include raid expansion, rebuild of failed drives, different RAID6 geometries, ... /dev/md0 and the underlying block devices contents were checked with sha256sum against an expected result of the unpatched module. Knock on wood so far no failures. 4) In between I was able to grab 10 older disk drives of different sizes and speeds and built a test rig. Simple RAID math should give 3read+3write I/Os for RMW and 7read+3write I/Os for RCW and thus a 66% improvement for write I/Os with a size smaller or equal to a single chunk. As you can see reality does not care about math but the effect is visible. Remember that larger arrays will show more speedups. 300 seconds random write with 8 threads 3,2TB (10*400GB) RAID6 64K chunk without spare group_thread_cnt=4 bsize rmw_level=1 rmw_level=0 rmw_level=1 rmw_level=0 skip_copy=1 skip_copy=1 skip_copy=0 skip_copy=0 4K 115 KB/s 141 KB/s 165 KB/s 140 KB/s 8K 225 KB/s 275 KB/s 324 KB/s 274 KB/s 16K 434 KB/s 536 KB/s 640 KB/s 534 KB/s 32K 751 KB/s 1,051 KB/s 1,234 KB/s 1,045 KB/s 64K 1,339 KB/s 1,958 KB/s 2,282 KB/s 1,962 KB/s 128K 2,673 KB/s 3,862 KB/s 4,113 KB/s 3,898 KB/s 256K 7,685 KB/s 7,539 KB/s 7,557 KB/s 7,638 KB/s 512K 19,556 KB/s 19,558 KB/s 19,652 KB/s 19,688 Kb/s Thanks Neil for your support. Markus
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