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According to How to remove a drive from a non-standard 2-drive RAID 5 array?, "with mdadm, a 2 drive RAID 5 is binary identical to a RAID1". If that is so, what's the mdadm command line for reshaping a 2-disk RAID1 into a degraded 2-disk RAID5 and what are the consequences [in the theoretical case of none of the drives ever failing]?

Also, what advantages and/or disadvantages does going that route and then adding two more disks, have over first breaking the RAID1 up, then creating a new RAID5 from its first disk, plus its original second disk and two new disks?

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Raid5 requires 3+ disks, so this won't be possible. https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5

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  • I'm not talking about generic RAID. I'm talking specifically about mdadm, where there are two sources (the one in my question as well as neil.brown.name/blog/20090817000931 saying that specifically this is possible). I'll gladly accept your answer if you can provide a definitive source disproving my two sources.
    – Sixtyfive
    Commented Jun 16, 2021 at 16:04
  • @Sixtyfive Your source specifically mentions REDUCING the number of devices in an existing array. Not creating a new array already reduced...Although I only mention this after a quick scan of your source and am not any sort of authority on disk arrays. EDIT: Also mentions that the feature is not working on RAID 5 and might be working somewhere some sunny day
    – mishan
    Commented Jun 16, 2021 at 16:09
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    I'm guessing the result of that 2-disk conversion would be a "degraded" RAID5 array (i.e. the same as if a normal RAID5 had lost a disk), which then needs to be brought back to normal by adding the 3rd (parity) disk.
    – grawity_u1686
    Commented Jun 16, 2021 at 16:09
  • thank you, added the word "degraded" to my question to indicate that i'm aware of this not being "proper".
    – Sixtyfive
    Commented Jun 16, 2021 at 16:12

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