Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 14 Aug 2012 17:49:52 -0700 | Subject | O_DIRECT to md raid 6 is slow |
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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.
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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