Re: O_DIRECT to md raid 6 is slow

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On 2012-08-15 08:49 Andy Lutomirski <luto@xxxxxxxxxxxxxx> Wrote:
>If I do:
># dd if=/dev/zero of=/dev/md0p1 bs=8M
>then iostat -m 5 says:
>
>avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>           0.00    0.00   26.88   35.27    0.00   37.85
>
>Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
>sdb             265.20         1.16        54.79          5        273
>sdc             266.20         1.47        54.73          7        273
>sdd             264.20         1.38        54.54          6        272
>sdf             286.00         1.84        54.74          9        273
>sde             266.60         1.04        54.75          5        273
>sdg             265.00         1.02        54.74          5        273
>md0           55808.00         0.00       218.00          0       1090
>
>If I do:
># dd if=/dev/zero of=/dev/md0p1 bs=8M oflag=direct
>then iostat -m 5 says:
>avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>           0.00    0.00   11.70   12.94    0.00   75.36
>
>Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
>sdb             831.00         8.58        30.42         42        152
>sdc             832.80         8.05        29.99         40        149
>sdd             832.00         9.10        29.78         45        148
>sdf             838.40         9.11        29.72         45        148
>sde             828.80         7.91        29.79         39        148
>sdg             850.80         8.00        30.18         40        150
>md0            1012.60         0.00       101.27          0        506
>
>It looks like md isn't recognizing that I'm writing whole stripes when
>I'm in O_DIRECT mode.
>
kernel version?

>--Andy
>
>-- 
>Andy Lutomirski
>AMA Capital Management, LLC
>--
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