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Old 02-09-2017, 03:19 PM   #1
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mdadm raid10 with 6 drives or more

I'm experimenting a lot with mdadm lately and finding that regardless of how many drives I use, e.g. if I create a --level=10 with 6 or even 8 drives, it will only create 2 mirrored sets like it does with 4 drives where that's the only possible option. Example follows.

Command used:
mdadm --create /dev/md1 --level=10 --raid-devices=8 /dev/{sdc,sdd,sde,sdf,sdg,sdh,sdi,sdj}
Number   Major    Minor   RaidDevice  State
       0       8       32        0      active sync set-A   /dev/sdc
       1       8       48        1      active sync set-B   /dev/sdd
       2       8       64        2      active sync set-A   /dev/sde
       3       8       80        3      active sync set-B   /dev/sdf
       4       8       96        4      active sync set-A   /dev/sdg
       5       8      112        5      active sync set-B   /dev/sdh
       6       8      128        6      active sync set-A   /dev/sdi
       7       8      144        7      active sync set-B   /dev/sdj
Do I need to use nesting to achieve a stripe of 2x mirrors? Am I reading the result wrong - are they separate sets? (which would be very confusing and unclear). I'm trying to figure out if I'm missing a parameter or if this is expected behavior unless going nested.


Last edited by thirdbird; 02-09-2017 at 03:23 PM.
Old 02-09-2017, 07:01 PM   #2
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That is what RAID10 is - RAID1 (a stripe set) + 0 (mirroring). Anything using/mirroring less than the full stripe set will not be (point-in-time) synching all the data.

I don't know what you mean by "nested" - alter the above printout to elucidate me pls.
Old 02-09-2017, 07:13 PM   #3
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This would be nested 1+0 where you can actually define how many drives are in each mirrored set that's being striped.

# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/{sda,sdb}
# mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/{sdc,sdd}
# mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/{sde,sdf}
# mdadm --create /dev/md3 --run --level=0 --raid-devices=3 /dev/{md0,md1,md2}
Which would look like this:
M	M | S | M	M | S | M	M |
Opposed to the previous command in my OP which gives 2 mirrored sets regardless of raid devices defined, which would look like this if there was 6 drives:
M	M	M | S | M	M	M |
So my question remains. Am I missing an obvious parameter or something to customize the amount of mirrored sets when using --level=10 or do you need to nest arrays to achieve the desired set ranges.


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