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I have three drives in my current system (all 1TB). The current setup is:

sdb - raid0 member (failing soon)
sdd - raid0 member
sde - backup

I'd like to change it so all three drives are part of a raid 1e (mdadm raid 10,n2) my main concern now is reliability, with space second, and speed as an appreciated bonus.

Looking at this manual I see I can create a "degraded" array (in this instance, with sdb and sdd) and copy the data onto it, and then add the third drive. It doesn't say anything about complex raid10 modes though. Data-wise raid 10n2 expecting 3 drives can exist with only 2, although I'm wondering if mdadm won't let me actually build and use a 10n2 array with a missing drive.

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I tried it with

mdadm --create /dev/md0 -n 3 -l 10 -p n2 /dev/sda2 /dev/sdb2 missing
...
mdadm md0 -a /dev/sdc2

and it seemed to work.

Since it operated in degraded mode I can assume data is mirrored, and array device size is 150% the size of a drive as expected.

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  • It worked for me with -n 2 -l 10 as well. This created a degraded mirror array.
    – Robert Cutajar
    Commented Oct 29, 2019 at 20:07

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