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Is it bad practice to run a 3 copy MD RAID10 array on two drives with a missing drive in place of a 2 copy MD RAID10 array on two drives to allow for the addition of a third drive in the future?

We are migrating servers and I will build a new array on the new server. The plan is to build a 3 copy array with a missing drive and leave it this way possibly for months to allow for the addition of a third drive in the future. Is this equivalent to running a 2 copy array or are there caveats to this?

Our current array is a 2 copy MD RAID10 array on two drives. We would like to add another level of redundancy but we do not have the third drive yet or the tray for it and nothing is pressing us make the purchase to implement this immediately.

  • A RAID10 is not possible with 2 HDDs, you need at least 4 HDDs for a RAID10. In general it's possible to start an array in degraded state, but I wouldn't recommend this. RAID10 means that you have a RAID0 built from several RAID1 arrays. If you start with one degraded RAID1 and that disk fails, all your RAID10 data is trashed. – Thomas Jan 8 '17 at 12:16
  • @Thomas MD RAID10 differs from true nested RAID1+0; a 2 drive MD RAID10 is possible. See suse.com/documentation/sles10/stor_admin/data/… or any other MDADM documentation you can find. In our case, a 2 drive MD RAID 1 had almost half the performance of our current 2 drive MD RAID 10 so we run it as is. Further, you can create any number of copies you want up to the number of devices; hence the question about a 3 copy 3 drive MD RAID10 created with a missing device. – Damon Jan 8 '17 at 17:06
  • thanks for updating. Didn't know that the layout of RAID10 is so different with mdadm. – Thomas Jan 8 '17 at 17:32
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I think the biggest loss would be that you lose the ability to easily tell if the array is broken (due to a failed drive) or not.

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