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On my new 24 disk array I get 900 MB/s of raw read or write using dd to all the disks.

When I set the disks up as a 24 disk RAID6 I get 400 MB/s write and 600 MB/s read. It seems to be due to checksuming, as I have a single process (md0_raid6) taking up 100% of one CPU.

# uname -a
Linux server 3.2.0-0.bpo.1-amd64 #1 SMP Sat Feb 11 08:41:32 UTC 2012 x86_64 GNU/Linux

I seem to remember back in kernel 2.6.x the kernel printed that it could do 8000 MB/s of RAID6 checksumming. It therefore surprises me that md0_raid6 is even close to 100%. I would have expected it to be around 10%.

Edit:

Found this in /var/log/messages:

Mar 13 16:02:42 server kernel: [   34.712039] raid6: int64x1   2125 MB/s
Mar 13 16:02:42 server kernel: [   34.780043] raid6: int64x2   2149 MB/s
Mar 13 16:02:42 server kernel: [   34.848049] raid6: int64x4   1832 MB/s
Mar 13 16:02:42 server kernel: [   34.916047] raid6: int64x8   1569 MB/s
Mar 13 16:02:42 server kernel: [   34.984041] raid6: sse2x1    3186 MB/s
Mar 13 16:02:42 server kernel: [   35.052031] raid6: sse2x2    5272 MB/s
Mar 13 16:02:42 server kernel: [   35.120032] raid6: sse2x4    6196 MB/s
Mar 13 16:02:42 server kernel: [   35.120035] raid6: using algorithm sse2x4 (6196 MB/s)
Mar 13 16:02:42 server kernel: [   35.144209] md: raid6 personality registered for level 6
Mar 13 16:02:42 server kernel: [   35.144212] md: raid5 personality registered for level 5
Mar 13 16:02:42 server kernel: [   35.144214] md: raid4 personality registered for level 4

I naïvely believed this meant that I could checksum 6196 MBytes/s.

Why does it take 100% and not around 10%?

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You waited for the initial sync to complete right? –  Zoredache Mar 13 '12 at 19:25
    
Yes. Even if I did not the sync would at most perform at 900 MBytes/s (the theoretical max speed as measured by dd), and 900 MBytes/s should again use around 10% CPU. –  Ole Tange Mar 13 '12 at 19:34
1  
Where's the question? :) –  jmtd Mar 14 '12 at 16:38
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