Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 15 Aug 2012 10:57:40 -0700 | Subject | Re: O_DIRECT to md raid 6 is slow |
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On Wed, Aug 15, 2012 at 4:50 AM, John Robinson <john.robinson@anonymous.org.uk> wrote: > On 15/08/2012 01:49, Andy Lutomirski wrote: >> >> If I do: >> # dd if=/dev/zero of=/dev/md0p1 bs=8M > > [...] > >> It looks like md isn't recognizing that I'm writing whole stripes when >> I'm in O_DIRECT mode. > > > I see your md device is partitioned. Is the partition itself stripe-aligned?
Crud.
md0 : active raid6 sdg1[5] sdf1[4] sde1[3] sdd1[2] sdc1[1] sdb1[0] 11720536064 blocks super 1.2 level 6, 512k chunk, algorithm 2 [6/6] [UUUUUU]
IIUC this means that I/O should be aligned on 2MB boundaries (512k chunk * 4 non-parity disks). gdisk put my partition on a 2048 sector (i.e. 1MB) boundary.
Sadly, /sys/block/md0/md0p1/alignment_offset reports 0 (instead of 1MB).
Fixing this has no effect, though.
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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