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Please, tell me if it is possibile to create RAID10 (near layout) with odd drives and then adding even(th) disk to that existing RAID10.

  • TOTAL: 4* 2TB drives (sdc,sdd,sde,sdf)
  • FREE: 3 of them (sdd,sde,sdf) are empty / unpartitioned; ready to RAID
  • IN-USE: 1 HDD (sdc) has data in it


  1. mdadm or lvm creation of RAID10 (near layout) using initially 3* 2TB HDDs (sdd1,sde1,sdf1)
  2. format RAID10 with ext4
  3. move all data from sdc1 into the newly created RAID10 (sdd1+sde1+sdf1)
  4. prepare sdc into sdc1
  5. mdadm or lvm adding sdc1 to the existing RAID10 (sdd1+sde1+sdf1)
  6. mount RAID10 into /somewhere/mystuff via /etc/fstab

Thanks!

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Yes, you can do it this way. I'm not convinced that a layout of sdd sde sdf missing RAID10 gives you any more security than creating a RAID0 first (sdd sde) and converting (sdd + sdf, sdc + sde), but that's an academic discussion. You could do it either way.

mdadm uses the special keyword "missing" in place of a device name to create/manage degraded arrays. So when you create the array, you would run mdadm --create /dev/md0 --level=10 --raid-devices=4 /dev/sdd1 /dev/sde1 /dev/sdf1 missing. Once the data has copied over, you can simply add the remaining disk by running mdadm --add /dev/md0 /dev/sdc1.

I highly recommend you create your array on top of partitions of type "Linux RAID" (0xfd00 or A19D880F-05FC-4D3B-A006-743F0F84911E) rather than on the raw devices.

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  • just for sake of "philosophy" dialog I'm curious ---> Why are you teling me about sdd sde sdf missing RAID1, even if asked infos bout RAID10? Also, are you telling me that with mdadm I could create a manual nested-RAID by first creating two RAID0 and then adding them manually to one RAID1? -- last "philosophic" question: will you go with RAID on devices sdc sdd sde sdf? Or with their partitions sdc1 sdc2 sdd1 sdd2 sde1 sde2 sdf1 sdf2?
    – T. Caio
    Jul 3, 2020 at 17:59
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    @T.Caio 1) Sorry, that was a typo: RAID1 should read RAID10 (I've made the edit). 2) You could in theory create a nested RAID like you suggest., but I don't recommend it. 3) I always put RAID on partitions, never on raw devices. It makes it easier to move to smaller/larger devices in the future, and it really helps with auto-detection/auto-assembly/auto-mounting.
    – squircle
    Jul 3, 2020 at 18:08
  • OK, as you pointed out in the original answer. Good! ;) Then'll be up to my decision if I want to create a single partition (eg: totalsize-128MiB) for each disk or to split them with multiple partitions... But that's another story.
    – T. Caio
    Jul 3, 2020 at 18:17
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    @T.Caio You shouldn't add multiple partitions on the same device to a single array (or even multiple arrays). If you want to divide the space up further, I'd suggest using LVM.
    – squircle
    Jul 3, 2020 at 18:19
  • Gothca. Well: I'll go with mdadm, good. ;)
    – T. Caio
    Jul 3, 2020 at 18:22

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